Determining Coalbed Methane Production and Composition from Individual Stacked Coal Seams in a Multi-Zone Completed Gas Well

energies

Communication Determining Coalbed Methane Production and Composition from Individual Stacked Coal Seams in a Multi-Zone Completed Gas Well

Nino Ripepi 1,2, Kyle Louk 2, Joseph Amante 2, Charlies Schlosser 2, Xu Tang 2,* ID and Ellen Gilliland 1,2

1 Department of Mining and Minerals Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24060, USA; [email protected] (N.R.); [email protected] (E.G.) 2 Virginia Center for Coal and Energy Research Center, Virginia Polytechnic Institute and State University, Blacksburg, VA 24060, USA; [email protected] (K.L.); [email protected] (J.A.); [email protected] (C.S.) * Correspondence: [email protected]; Tel.: +1-(540)-998-7174

Received: 28 August 2017; Accepted: 27 September 2017; Published: 2 October 2017

Abstract: This work proposes a novel and cost-effective approach to determine coalbed methane (CBM) production and composition from individual coal seams in a multi-zone completed CBM well. The novel method uses water to cover individual coal seams in a low pressure CBM well followed by an Echometer fluid level survey to determine the water level. Corresponding gas flow measurements and natural gas chromatography analysis are used to determine gas production and composition from unique zones. A field test using this technology is conducted in Central Appalachia for a multi-zone CBM well containing 18 coal seams. Test results show that the shallow coal seams contribute the majority of the total CBM production in this multi-zone well, and the deeper coal seams contain more heavy hydrocarbons like ethane and propane.

Keywords: multi-zone CBM well; CBM production; CBM composition

1. Introduction A typical coalbed methane (CBM) well in Central Appalachia is produced from multiple stacked thin coal seams over approximately 1000 feet of stratigraphic section [1–3]. These coal seams are typically grouped together in zones and hydraulically fractured with a nitrogen-foam completion strategy in order to increase the permeability of the coal to allow for commercial gas flow [4,5]. Even though the gas content and composition for each coal seam could be known in advance through desorption testing in the exploration stage, it is challenging to determine the contribution of each coal seam or zone as part of the total flow from the CBM well during the production stage [6,7]. This can be attributed to the complex nature of in situ coal reservoir conditions such as in situ stress, coal permeability, coal rank, the saturation status, and heterogeneous physical properties of coal [6–8]. The CBM in deeper coal seams may not be produced as easily because of the low permeability induced by high in situ vertical stress when compared with shallow coal seams [9]. Furthermore, the direction, length, and proppant placement from the hydraulic fracture stimulation is not known and likely varies significantly by coal seam. In fact, it has been shown that during zone stimulation of stacked coal seams, some of the seams do not take any of the proppant [4]. The application of production logging by use of a spinner survey and production data analysis could be very helpful in determining the contribution of each coal seam [3,10–12], but it is very difficult because of low pressure and flow rates, as well as expensive because a downhole pump would need to be removed and water swabbed from the wellbore. Therefore, developing a feasible and cost-effective method is needed in order to determine CBM production and composition from individual coal seams in multi-zone CBM wells.

Energies 2017, 10, 1533; doi:10.3390/en10101533 www.mdpi.com/journal/energies Energies 2017, 10, 1533 2 of 8

Additionally, geological sequestration of carbon dioxide in coal seams is of interest because enhanced gas recovery due to injected carbon dioxide could reduce the total cost of carbon sequestration [12,13]. Researchers from Virginia Tech have injected 14,000 tons of carbon dioxide into three CBM wells in Central Appalachia to investigate the feasibility of injecting carbon dioxide into unmineable coal seams while simultaneously enhancing CBM recovery [14–17]. One of the objectives of this project is to track the migration of carbon dioxide throughout the injection and post-injection phases, including returning the legacy wells to production. Since the carbon dioxide is injected into multiple stacked coal seams, how the injected carbon dioxide is stored in each coal seam is still under investigation [17]. In order to answer this question, a cost-effective approach to determine the amount of the injected carbon dioxide produced from each coal seam is needed, and this technique will be applied when the three injection wells are reconverted to CBM production wells. A typical multi-zone CBM well in Central Appalachia has 4.5 inch casing cemented through the wellbore with perforations shot into each of the stacked coal seams so that gas and water can flow into the well. Hydraulic fracture treatments are typically done by grouping coal seams in three or four zones and stimulating with 75% by volume (quality) of nitrogen gas foamed as a carrier fluid to transport proppant in order to break down the coals and place sand to hold open the fractures. After hydraulic fracture treatments and flowback, CBM wells in this basin are configured with a downhole pump that can produce water up a 2-3/8 inch tubing while gas is simultaneously produced up the annulus of the tubing. Water is pumped from the well at predetermined intervals and lengths of time throughout the day in order to keep water from covering any of the coal seams that are producing gas. The gas production is metered at the surface by calculating the pressure differential across an orifice plate. Gas production from these wells is typically at very low pressures; 1–5 psi over atmosphere is common. This work proposes a novel and cost-effective method to determine the gas production and composition from individual coal seams in a multi-zone CBM well by combining an Echometer fluid level survey while systematically killing the production from the well. A field test using this method is conducted in Central Appalachia for a multi-zone CBM well containing 18 unique coal seams. The gas production and composition for coal seams of interests in this multi-zone test well are analyzed.

2. Theory, Method, and Field Test

2.1. Theory In water-saturated coals, the production of CBM is directly related to the drainage of water from the coal seam. Dewatering is usually required to depressurize the seam and enable the gas to flow out of the coal and into the production well [6,7,18,19]. Simply speaking, a water pump is placed at the bottom of the CBM well to dewater the well, followed by CBM production. The well kill test is accomplished by injecting water in the annulus of the wells in order to slowly cover producing seams and thus kill the production from those covered coal seams because the hydrostatic pressure in the column of water is greater than the low coal seam gas pressure. An Echometer fluid level survey is taken immediately after each water injection phase to determine the water level. The CBM production and composition for these seams can be determined by the flow rate and gas composition analysis of the produced gas from the well head. The theory of this novel well kill test allows the production from each coal seam to be separated from a multi-zone CBM well one by one, which paves the way for determining the gas contribution of each unique coal seam or groups of coal seams if the seams are located in close proximity to each other stratigraphically. The Echometer can compute the distance down-hole to the surface of the liquid in between the annulus of the tubing and the casing. This is accomplished by using the speed of sound of the carrier gas based on the principle of creating a sound wave that travels from the surface, down the casing, reflects off the surface of the fluid, and bounces back to the top of the well where the sound wave is picked up by a sensitive pressure transducer [20,21]. By combining a gas flowmeter and natural gas chromatography analysis of the produced gas from the CBM well, the gas production and composition contributed by each coal seam can be determined. Energies 2017, 10, 1533 3 of 8

Energies 2017, 10, 1533 3 of 8 2.2. Method 2.2. Method The general test procedure is illustrated by a CBM well containing three coal seams labeled A, B, and C. InThe step general (i), the test three procedure coal seams is illustrated are producing by a CBM at the well same containing time. D1 three is obtained coal seams by an labeled Echometer A, B, and C. In step (i), the three coal seams are producing at the same time. D1 is obtained by an fluid level survey as the baseline. Accumulated gas production is recorded as Vc, and the natural Echometer fluid level survey as the baseline. Accumulated gas production is recorded as Vc, and the gas is sampled from a stable production gas flow to determine each composition as M1c, M2c,M3c, natural gas is sampled from a stable production gas flow to determine each composition as M1c, M2c, ... ,Mnc, where n represents the total number of components and the unit is percentage (%). In step M3c,…,Mnc, where n represents the total number of components and the unit is percentage (%). In step (ii), a certain amount of water is injected into the CBM well through a water truck followed by an (ii), a certain amount of water is injected into the CBM well through a water truck followed by an Echometer fluid level survey until coal seam C is saturated and the water level is D2. It is worth noting Echometer fluid level survey until coal seam C is saturated and the water level is D2. It is worth thatnoting multiple that Echometermultiple Echometer surveys maysurveys be neededmay be inneeded order in to order make to sure make the sure water the wellwater is well stable, is stable, because somebecause dry coal some seams dry coal may seams vacuum may in vacuum the injected in the water. injected When water. the gasWhen flow the is gas stable, flow the is accumulatedstable, the gasaccumulated production isgas recorded, production Vb ,is and recorded, the composition Vb, and the of composition gas sample of is gas analyzed sample as is M analyzed1b,M2b,M as 3bM,1b..., , MnbM.2b In, M step3b,…, (iii), Mnb. an In additionalstep (iii), an slug additional of water slug is of injected water is to injected saturate to coal saturate seam coal B following seam B following the same procedurethe samein procedure step (ii), in and step D3 (ii), is and obtained D3 is obtained by an Echometer by an Echometer fluid level fluid survey. level survey. When When the gas the flow gas is stable,flow theis stable, gas flow the gas is recorded, flow is recorded, Va, and V thea, and composition the composition of gas of sample gas sample is analyzed is analyzed as M as1a ,MM1a2a, M,M2a, 3a, ...M,M3a,…,na. M Byna. combining By combining these these three three steps, steps, the the gas gas production production andand composition contributed contributed by by each each coalcoal seam seam can can be be determined. determined. UtilizingUtilizing the the first first and and second second test test case case inin FigureFigure1 1 asas anan example, the the gas gas production production from from Group Group C coalC coal seams seams can can be be calculated calculated by by the the Equation Equation (1).(1).

FigureFigure 1. Simpliﬁed1. Simplified procedures procedures for for determining determining gasgas productionproduction and and composition composition of of each each coal coal seam seam fromfrom a multi-seam a multi-seam coalbed coalbed methane methane (CBM) (CBM) well. well.

Gas production of Group C coal seams (Gc), which is always positive, Gas production of Group C coal seams (G ), which is always positive, = c − Gc Vc Vb (1) Gc = Vc − Vb (1) The gas composition from Group C can be calculated by, The gas composition from Group C can⋅ be calculated− ⋅ by, Vc M nc Vb M nc Mn = (2) Gc V −V Vc ·cMnc b− Vb · Mnc MnGc = (2) Vc − Vb 2.3. Field Tests 2.3. Field Tests The coal field for this study is located in Buchanan County, Virginia, where there are typically 15–20The coal coal seams ﬁeld for completed this study from is locatedthe Pocahontas in Buchanan and Lee County, formations, Virginia, averaging where there0.3 m are (1.0 typically ft) in 15–20thickness, coal seams for net completed reservoir fromthicknesses the Pocahontas of approximately and Lee 4.5–6.0 formations, m (15–20 averaging ft) [14]. 0.3 The m (1.0general ft) in thickness,stratigraphic for netcolumn reservoir for coals thicknesses completed of in approximatelythis study well is 4.5–6.0 shown m in (15–20Figure 2 ft) and [14 consists]. The generalof 18 stratigraphicdifferent coal column seams. for coals completed in this study well is shown in Figure2 and consists of 18 different coal seams.

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FigureFigure 2. 2.Coal Coal seam seam thickness thickness in in the the test test well. well.

The field test consists of five cyclic sampling tests performed by the proposed method. Both The field test consists of five cyclic sampling tests performed by the proposed method. Both collar counting and acoustic velocity tests are used with Echometer software to accurately determine collar counting and acoustic velocity tests are used with Echometer software to accurately determine the water level in the CBM well. The instantaneous accumulated gas production flow is obtained the water level in the CBM well. The instantaneous accumulated gas production flow is obtained from a total flow meter at the test well which is manufactured by ABB Inc. The CBM sample is from a total flow meter at the test well which is manufactured by ABB Inc. The CBM sample is obtained thorough a gas sample cylinder and sampled CBM is analyzed by a portable Natural Gas obtained thorough a gas sample cylinder and sampled CBM is analyzed by a portable Natural Gas Chromatograph (NGC). Chromatograph (NGC). The experiment started with a stable baseline for both the gas composition and the liquid level. The experiment started with a stable baseline for both the gas composition and the liquid level. Water was then injected into the well in varying intervals to increase the hydrostatic pressure to kill Water was then injected into the well in varying intervals to increase the hydrostatic pressure to kill coal seams at different depths to obtain new and stable gas compositions and flow rates. This cycle coal seams at different depths to obtain new and stable gas compositions and flow rates. This cycle was repeated until the well was effectively killed and the flowrate was zero. Detailed information was repeated until the well was effectively killed and the flowrate was zero. Detailed information about the Echometer liquid level measurement and natural gas sampling and analysis method are about the Echometer liquid level measurement and natural gas sampling and analysis method are referred to the Supplemental Materials. referred to the Supplemental Materials.

3.3. Test Test Results Results

3.1.3.1. Covered Covered Coal Coal Seams Seams and and Production Production Behavior Behavior BasedBased on on the the Echometer Echometer test test results, results, the the covered covered coal coal seams seams can can be be classified classified into into five five individual individual groups,groups, G1–G5, G1–G5, and and at at five five different different water water level level depths, depths, 2150 2150 ft, ft, 2017 2017 ft, ft, 1965 1965 ft, ft, 1858 1858 ft, ft, and and 1486 1486 ft, ft, asas shown shown in in Figure Figure3a. 3a. The The net net coal coal seam seam thickness thickness in in each each individual individual group group of of coals coals are are different, different, as as shownshown in in Figure Figure3b. 3b. It isIt is worth worth noting noting that that each each phase phase of of the the test test lasts lasts from from 30–60 30–60 min, min, which which enables enables enoughenough time time to to inject inject water, water, stabilize stabilize gas gas flow, flow, take take an an Echometer Echometer reading, reading, and and sample sample gas gas for for gas gas compositioncomposition analysis. analysis. It isIt extremelyis extremely important important to stabilize to stabilize the gas the flow gas asflow pressure as pressure is allowed is allowed to build to duringbuild theduring water the injection water injection and Echometer and Echometer test. test. TheThe accumulated accumulated gas gas production production for eachfor each phase phase of the of well the testwell shows test shows that when that waterwhen killswater more kills coalmore seams, coal theseams, accumulated the accumulated gas production gas production decreases, decreases, as shown inas Figure shown4a. in The Figure gas production4a. The gas fromproduction each group from shows each thatgroup the shows shallow that groups, the sha G1llow and groups, G2, produce G1 and more G2, gas produce when compared more gas withwhen thatcompared of the deep with G3, that G4, of andthe deep G5 groups G3, G4, in and Figure G54 groub. Theps gasin Figure production 4b. The per gas unit production coal thickness per unit from coal eachthickness coal group from showseach coal that group G3 has shows the highestthat G3 gas has production the highestfollowed gas production by G2, followed G1, G5, and by G2, G4 inG1, FigureG5, and4c. BasedG4 in onFigure the above 4c. Based observations, on the above it can observations, be concluded thatit can coal be seams concluded in shallow that coal groups seams (G1, in G2,shallow and G3) groups contribute (G1, G2, more and to G3) the contribute total gas productionmore to the thantotal thegas deepproduction groups than of coals the deep (G4 andgroups G5). of Unfortunately,coals (G4 and itG5). is difficult Unfortunately, to ascertain it is difficult the correct to ascertain reasons behindthe correct this reasons observation behind because this observation there are abecause number there of affecting are a number factors, of such affecting as coal factors, permeability, such as in coal situ permeability, stress, gas content, in situ gasstress, pressure, gas content, and watergas pressure, saturation and status, water in saturation this real scenario status, thatin this are real coupled scenario and that complex. are coupled and complex.

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FigureFigureFigure 3. 3. Water (3.a )Water Water levels levels levels from from from Echometer Echometer Echometer tests tests tests; ( a(a) )and and (b) net netnet thickness thicknessthickness of of water-covered water-covered coal coal coal seams seams seams in in in each each each Figure 3. Water levels from Echometer tests (a) and net thickness of water-covered coal seams in each test.test G1–G5 (b). G1–G5 represents represents ﬁve individual five indivi groupsdual ofgroups coals forof eachcoals test,for blueeach colortest, representsblue color water/waterrepresents test test(b). ( bG1–G5). G1– G5represents represents five five indivi individualdual groups groups of of coals coals forfor eacheach test,test, blueblue colorcolor represents levelwater/water in the CBM level well, in the and CBM black well, bar and represents black bar coal represents seams (not coal to seams scale). (not to scale). water/waterwater/water level level in thein the CBM CBM well, well, and and black black bar bar represents represents coal coal seams seams (not (not toto scale).scale).

Figure 4. Gas production from each well kill test: (a) represents accumulative gas production from FigureFigureshallowFigure 4. 4. Gas Gas4. coal Gas production production seams production to fromdeep from from eachcoal each each seams,well well well kill kill kill(b test:) test:gas ( (a aproduction())a )represents represents represents at accumulative accumulativeaccumulativeeach coal seam gas gas interval, productionproduction and fromfrom from(c) shallow coal seams to deep coal seams, (b) gas production at each coal seam interval, and (c) shallowshallowrepresents coalcoal seams theseams gas to productionto deep deep coal coal seams;per seams,unit (b coal) gas(b thickness) productiongas production for each at each intervals at coal each seam coal interval; seam andinterval, (c) represents and (c) represents the gas production per unit coal thickness for each intervals representsthe gas production the gas production per unit coal per thickness unit coal for thickness each intervals. for each intervals 3.2. CBM Composition in Each Tested Group 3.2. CBM Composition in Each Tested Group 3.2.3.2. CBM CBMFor Composition Composition each phase in inof Eachthe Each well Tested Tested kill Group test, composition analysis of the produced gas shows that methane For each phase of the well kill test, composition analysis of the produced gas shows that methane contentFor each dominates phase of the wellnatural kill test,gas compositioncontent, aver analysisaging around of the produced98% in Figure gas shows 5a. The that volumemethane contentFor each dominates phase of the wellnatural kill test,gas compositioncontent, aver analysisaging around of the produced98% in Figure gas shows 5a. The that methanevolume contentpercentage dominates of heavier the hydrocarbonsnatural gas content,(ethane, propaverane,aging and around butane) 98%shows in an Figure increasing 5a. trendThe volumewhen contentpercentage dominates of heavier the natural hydrocarbons gas content, (ethane, averaging propane, around and 98%butane) in Figure shows5 a.an The increasing volume trend percentage when percentagedeep coal of seams heavier produce, hydrocarbons which indicates (ethane, that prop heavane,ier and hydrocarbons butane) shows come an from increasing deeper coal trend seams, when of heavierdeep coal hydrocarbons seams produce, (ethane, which propane,indicates andthat butane)heavier hydrocarbons shows an increasing come from trend deeper when coal deep seams, coal deepas coalshown seams in Figure produce, 5b. which indicates that heavier hydrocarbons come from deeper coal seams, seamsas shown produce, in Figure which 5b. indicates that heavier hydrocarbons come from deeper coal seams, as shown in as shown in Figure 5b. Figure5b.

Depth (ft) Depth Depth (ft) Depth

Depth (ft) Depth Figure 5. Gas composition of accumulated CBM sample: (a) represents gas composition of accumulated CBM sample from shallow seams to deep seams and (b) represents an enlarged section Figureof Figure 5. Gas (5a). composition of accumulated CBM sample: (a) represents gas composition of accumulatedFigure 5. CBMGas samplecomposition from shallowof accumulated seams to CBMdeep seamssample: and (a ()b )represents represents gas an enlargedcomposition section of Figure 5. Gas composition of accumulated CBM sample: (a) represents gas composition of accumulated of Figureaccumulated (5a). CBM sample from shallow seams to deep seams and (b) represents an enlarged section CBM sample from shallow seams to deep seams and (b) represents an enlarged section of Figure 5a. of Figure (5a).

Energies 2017, 10, 1533 6 of 8

Energies 2017, 10, 1533 6 of 8 According to the mass balance, the natural gas composition of each group can be obtained from According to the mass balance, the natural gas composition of each group can be obtained from the accumulated gas production data from each phase of the well kill test. The methane content of the accumulated gas production data from each phase of the well kill test. The methane content of each group also averages around 98%, as shown in Figure6a. It can also be seen that the deep groups, each group also averages around 98%, as shown in Figure 6a. It can also be seen that the deep groups, G1 and G2, have signiﬁcantly higher heavier hydrocarbons such as ethane, propane, and butane than G1 and G2, have significantly higher heavier hydrocarbons such as ethane, propane, and butane than the shallow groups, namely, G3, G4, and G5, as shown in Figure6b. the shallow groups, namely, G3, G4, and G5, as shown in Figure 6b.

FigureFigure 6. Gas 6. composition Gas composition for each for each coal seamcoal seam interval interval at different at different depths: depths: (a) Gas (a) Gas composition composition for eachfor coal seameach interval coal seam at differentinterval at depth, different and depth, (b) represents and (b) represents an enlarged an sectionenlarged of section Figure of 6a. Figure (6a).

4. Discussion and Outlook 4. Discussion and Outlook This work proposes a novel and cost-effective engineering approach to determine CBM This work proposes a novel and cost-effective engineering approach to determine CBM production production and composition from individual coal seams in multi-zone coalbed methane wells. This and composition from individual coal seams in multi-zone coalbed methane wells. This technology technology uses water to cover coal seams of interest in a multi-zone CBM well to kill their production, uses water to cover coal seams of interest in a multi-zone CBM well to kill their production, followed followed by an Echometer fluid level survey to determine the water level as well as the corresponding by an Echometer fluid level survey to determine the water level as well as the corresponding natural natural gas production measurement and gas composition analysis. The field test for a multi-zone CBM gas productionwell using this measurement approach reveals and gas that composition the produced analysis. CBM contains The field around test for 98% a multi-zone methane, the CBM shallow well usingcoal this seams approach contribute reveals the thatmajority the produced of the total CBM CBM contains production around in this 98% multi-zone methane, well, the and shallow the deep coal seamscoal contribute seams contain the majority more heavy of the hydrocarbons total CBM like production ethane and in thispropan multi-zonee. The proposed well, and approach the deep will coal be seamsapplied contain to track more produced heavy hydrocarbons carbon dioxide like from ethane an on-going and propane. huff-and-puff The proposed test. approach will be applied toThe track purpose produced of this carbon communication dioxide from is to disclose an on-going this latest huff-and-puff technology test. to engineers and scholars inThe the purpose natural gas of this industry communication and to inspire is to in-depth disclose modeling this latest and technology continued to research, engineers as and the scholarstopic in in thethis natural work has gas not industry been previously and to inspire studied. in-depth Determining modeling the and gas continued contribution research, from individual as the topic coal in thisseams work hasin multi-seam not been previouslyCBM wells studied.is extremely Determining important the because gas contribution this not only from influences individual the CBM coal seamsproduction in multi-seam forecast, CBM but wellsalso affects is extremely the compos importantition of because the produced this not CBM. only influencesThis work thenot CBMonly productionprovides forecast, a preliminary but also solution affects thefor compositionthis problem, of but the also produced discloses CBM. some This more work interesting not only providesscientific a preliminaryquestions which solution warrant for this further problem, investigation. but also discloses Do deep some coal moreseams interesting always contain scientific more questions heavier whichhydrocarbons warrant further than investigation.shallow coal seams? Do deep Does coal th seamse gas composition always contain of produced more heavier CBM hydrocarbons change with thantime? shallow Why coal do deep seams? coal Does seams the contribute gas composition less gas production of produced in a CBM multi-zone change CBM with well? time? As Whyshallow do deepcoal coal seams seams are contribute depleted of less gas, gas will production the deep coal in se aams multi-zone contribute CBM more well? in the As future? shallow Is the coal nitrogen seams are depletedin the production of gas, willstream the from deep the coal fracture seams treatment? contribute If so, more is nitrogen in the future? produced Is from the nitrogen a specific in zone the productionrelated to stream flow rate from or the does fracture that change treatment? over time? If so, is nitrogen produced from a specific zone related to flow rateAll orof doesthese thatquestions change and over many time? more remain unclear, but could be solved by utilizing this technologyAll of these to questionsevaluate more and CBM many wells more as remainwell as by unclear, tracking but individual could be CBM solved wells by over utilizing time. thisThe authors believe that if a larger well kill dataset was put together for a group of CBM wells, one could technology to evaluate more CBM wells as well as by tracking individual CBM wells over time. analyze the produced gas for heavier hydrocarbons and estimate how much of the gas was being The authors believe that if a larger well kill dataset was put together for a group of CBM wells, one produced from the deeper versus the shallower coal seams. This data would allow a CBM operator could analyze the produced gas for heavier hydrocarbons and estimate how much of the gas was being to make better-informed decisions on which coal seams to target with respect to initial stimulation produced from the deeper versus the shallower coal seams. This data would allow a CBM operator to and if it would be profitable to re-stimulate coals or groups of coals that were previously not good make better-informed decisions on which coal seams to target with respect to initial stimulation and if producers. This also means if the majority of the gas is being produced early on in the life of the well it wouldfrom bethe profitable shallow seams, to re-stimulate then it might coals prove or groups to be more of coals economical that were to previously drill and complete not good those producers. seams

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This also means if the majority of the gas is being produced early on in the life of the well from the shallow seams, then it might prove to be more economical to drill and complete those seams versus spending the money to drill deeper and produce the deeper seams. In order to verify this hypothesis, one would need to repeat this test for a single well annually over the first five years of production of that well. Next, decline curves of production by seam could be created for that five-year period, and a full economic analysis could be performed that would include drilling, completion, and operating costs on a seam basis and the market price of gas [22]. Since heavier hydrocarbons such as propane and butane have higher calorific value than methane and a higher market value, this technology also empowers CBM operators to preferentially produce gases with heavier hydrocarbons as the exact gas composition is known. Alternatively, if CO2 is being produced in quantities that reduce the market price of the gas or require the use of expensive removal technologies, a CBM operator could identify the contribution of CO2 by seam and not complete that seam. The information disclosed from this technology could prove vital to a CBM operator who is trying to maximize profit.

Supplementary Materials: The following are available online at www.mdpi.com/1996-1073/10/10/1533/s1, Figure S1: Schematic (a) and real test (b) of Echometer liquid level survey; Figure S2: Acoustic test with reflection surface for depth; Figure S3: Collar counting for depth; Figure S4 Gas composition sampling and analysis facilities; Figure S5: ASTM Method D1945-96 Natural Gas Analysis; Figure S6 Sample NGC Chromatographic Report. Acknowledgments: This research was supported in part by the U.S. Department of Energy through the National Energy Technology Laboratory’s Program under Contract No. DE-FE0006827. Author Contributions: N.R. provided the main idea, designed and supervised the project; N.R., K. L., J. A. and C.S. conducted field and lab experiments; X.T., C.S. and N.R. analyzed the data and drafted the manuscript; and C.S. and E.G. reviewed and verified results. All authors reviewed the manuscript. Conflicts of Interest: The authors declare no conflicts of interest.

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