SEPTEMBER 2018
The “Better Business” Publication Serving the Exploration / Drilling / Production Industry Approach Optimizes Well Geosteering
By Christopher Viens When a change in total gamma is not based 3-D geosteering solution rather and Mark Tomlinson associated with an up or down movement than conventional 2-D geosteering soft- through stratigraphy, it indicates faulting, ware. By integrating multiple well and HOUSTON–Geosteering in horizontal a depositional anomaly, heterogeneity, or seismic surface inputs, this approach wells involves correlating logging data bedding that is not laterally continuous gives the geosteering geologist the infor- to a type log from a nearby offset well to or stratified. Because the fundamental mation to confidently resolve abrupt bed characterize the zone of interest. Corre- basis of geosteering is correlating to dip changes, identify faults, identify areas lating against a type log requires the bed- marker beds that have lateral continuity, of lateral continuity/discontinuity, identify ding thickness and gamma character of in situations where lateral continuity is stratified/unstratified zones and understand the target well to be close to that of the absent, only a low level of interpretation formation-related directional drilling tra- type log, which can be offset by miles in confidence can be achieved using tradi- jectory phenomena, etc. some cases. If not, the resulting geosteering tional correlation methods. interpretation will have unreasonable bed To maximize the benefits of azimuthal Laterally Continuous Bedding dips that do not accurately reflect the gamma imaging, a protocol has been de- In areas with laterally continuous target lithology being drilled, leaving the veloped to identify and handle challenging strata, the gamma character in the type geosteering geologist running multiple geosteering situations using a model- well typically will be present at the simultaneous interpretations in the hope that one will start to make sense as new data come in during drilling. FIGURE 1 Azimuthal gamma imaging, in con- Incorrect (Top) and Correct (Bottom) Geosteering Interpretations junction with continuous inclination meas- With Type Log (Right) urements, brings clarity to stratigraphic 1,000’ Measured Depth s ° position, bed dip, faults and zones where y
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Reproduced for DynaView with permission from The American Oil & Gas Reporter www.aogr.com FIGURE 2 bit. Surface deformation is propagated ahead Bed Counting Method to Track Stratigraphic Position of the bit with a corresponding change in projected synthetic petrophysical curves. This permits greater understanding of ex - 0 1 2 1 0 (__-1__ ) -2 -3 -4 -5 -6 -5 -4 -3 -2 -3 -2 -3 -2 pected properties ahead of the bit to mitigate 0 0 ) 2 I unnecessary steering changes based on mis - P A (
interpretations, and allows smoother and a
m more gradual steering adjustments to remain m a
G in the target zone while mitigating increased 0 wellbore tortuosity. 1,000’ MD This continuous updating with new data and correlations results in a high-quality +2 geomodel that can be used in laterally +1 0 continuous bedding to model the azimuthal gamma response along a projected well -1 path. As real-time gamma image data are -2 logged, a close match between the synthetic -3 azimuthal gamma image and measured -4 azimuthal image is a sign of an accurate -5 stratigraphic position interpretation. -6 This projection method reduces unnec - essary course corrections and increases landing point and throughout the well - curve because of its downward-cutting correlation confidence. To achieve an ac - bore’s lateral length. This type of bedding trajectory. curate synthetic image, a 3-D model is re - is ideal for geosteering since a proper in - quired to account for bed thickening/thinning terpretation will overlay the type log with Synthetic Data Between Wells and topographical changes. A close match little deviation. While these cases are When improved imaging visibility is between both images gives the geologist a ideal for geosteering, they are still subject combined with 3-D geomodeling incorpo - high level of confidence in the interpretation to ambiguity when utilizing a 90-foot rating on-the-fly correlation and updating, and the accuracy of the geomodel. When survey and only total gamma data. a higher level of correlation confidence encountering unexpected laterally discon - tinuous bedding, the 3-D model provides Figure 1 demonstrates how geosteering can be achieved. Geomodeling permits the greater confidence in maintaining trajectory interpretations can be ambiguous and creation of synthetic petrophysical data by until correlatable laterally continuous and highly subjective as a result of the position propagating data from all nearby wells, stratified features are encountered again. of 90-foot surveys, as well as a lack of accounting for thickening and thinning ac - Since azimuthal gamma image features up or down stratigraphic movement indi - cording to the geometry of surfaces within are a result of wellbore trajectory, bed cations from the total gamma measure - the model. These synthetic properties can dip angle and wellbore direction, to ac - ment. The type log at right shows a 15- be projected through the model and com - curately generate synthetic images that foot vertical profile of a zone. The inter - pared with actual data. Correlation adjusts honor measured images, utilizing a con - pretation at the top is based on 90-foot surface depths and thicknesses, resulting tinuous inclination-generated survey results surveys and total gamma data. In contrast, in a corresponding change of character in in a more accurate match. The relationship the lower interpretation uses continuous the synthetic curves, resulting in validation between bed dip, wellbore survey and inclination and azimuthal gamma imaging. of the geosteering interpretation. gamma image features are presented in The different stratigraphic position as in - A correctly interpreted correlation is re - Figure 3, which demonstrates discrepancies dicated by the numbering is a result of flected in high character coherence between between a 90-foot survey position (left) the definitive direction indication from the actual and synthetic curves behind the azimuthal gamma, and the continuous inclination curve bringing visibility to FIGURE 3 wellbore inclinations not characterized Bed Dip Calculation Utilizing 90-foot Surveys by the 90-foot surveys. Versus Continuous Inclination Figure 1 shows only 1,000 feet of meas - ured depth in a lateral. One can imagine how many incorrect interpretations may be made over the course of a 5,000-10,000 foot lateral simply because of data ambiguity if the proper measurements for accurate wellbore placement are not utilized. In laterally continuous zones without faulting or strata pinching, the bed counting method (Figure 2) can be performed to track stratigraphic positioning relative to the landing point. This is a powerful method for validating stratigraphic posi - tioning without actively performing a geosteering correlation. However, the method is not effective for landing a Special Report: Reservoir Modeling & EUR
and continuous survey position gamma Integrating well data and geological horizons graphic position, and in the event bedding image (right) for the same dip pick. In (using seismic horizons or synthetic surfaces is too chaotic for any regional or local this case, the continuous survey shows a tied to regional offsets) permits the propa - correlatable feature, the 3-D model can 2.1-degree difference in bed dip compared gation of offset information from multiple be used as a guide to maintain the well with the 90-foot survey calculation. wells to the actual drilled well location. path in the target lithology. The lack of wellbore inclination char - When encountering zones in a lateral Rapid interpretation of information acterization is one of the largest contributing wellbore where an azimuthal gamma image also is aided through data visualization factors to true vertical depth measurement identifies chaotic bedding or lateral dis - that can be extracted from a 3-D model. errors. This error presents itself in bed continuity, the 3-D model can be used to Contour maps of encountered horizons dip calculations when dip picking azimuthal project a well path ahead of the bit that will can account for lateral wellbore trajectory gamma image features, as well as across maintain the trajectory within the theoretical changes, and also assist in interpreting correlation cells in geosteering interpre - zone of interest until definable bedding or stratigraphic positional changes related tations. A lack of TVD characterization correlatable points are re-encountered. to a trajectory change. can lead to an inability to understand Through these zones of correlation uncer - For example, should a well drift az - local bed dips, as well as give bed bound - tainty, synthetic petrophysical curves can imuthally because of a lithologically or aries the appearance of being jagged or be created from a variety of offset wells to structural directional trend, a 2-D inter - having shifting surface angles with every identify any correlatable features that may pretation only would indicate the vertical new segment/correlation cell. Proper well - not be immediately apparent in a type log. transition out of a zone of interest. If the bore inclination characterization with con - The model-based approach eliminates well then is steered back to the desired tinuous inclination results in more realistic the need to carry out unnecessary trajectory azimuthal trajectory and an inclination bed dip calculations and removes much changes to locate a point of known strati - change is made to account for the inter - of the undulation often seen in geosteering interpretation cross-sections. FIGURE 4 Laterally Discontinuous Zones Wellbore Visualization in Structural Geological Context Higher-energy depositional environments often result in chaotic bedding, with little lateral continuity or stratification. In these environments, logging data characteristics exude little coherency from well to well, making correlation difficult and inaccurate. In marine depositional environments, distal and proximal depositions can be in direct stratigraphic contact with one another. As a result, small stratigraphic movements of the wellbore–whether from trajectory change, pinching out or faulting–can result in encountering lithology with very different depositional bedding characteristics. When using a total gamma measure - ment only for geosteering, the transition into a zone of laterally discontinuous FIGURE 5 bedding may not become apparent until Fully Integrated Dataset from 3-D Model the logging data cannot be correlated against the type log with any degree of certainly. At this point, correlation confi - dence is lost and various actions must be taken to try to identify where the wellbore is located stratigraphically. This adds drilling time, uncertainty and potential for lost pay zone exposure. All of this can be avoided through azimuthal gamma imaging’s enhanced visibility. Geosteering based within a 3-D model aids in understanding where the wellbore should be drilled when traditional correlation no longer can be carried out because of a lack of laterally continuous bedding from faulting, deformation strain or high-energy deposition. In all cases, geological com - plexity hinders accurate wellbore placement. Special Report: Reservoir Modeling & EUR
preted vertical transition, a greater-than- direction verification. • (7) Traditional overlay plots add required adjustment will be carried out • (4) The curtain view cross-section further verification to correlations. The and the well may be oversteered and exit backdrop is a synthetic gamma ray propa - red and orange arrows indicate regions the target on the opposite side. Visualizing gated through the 3-D model from the in the well where a traditional overlay this information, in conjunction with the offset well used for the synthetic image correlation deviates from the type log resulting effects on synthetic curve and generation for a visual stratigraphic image well and could cause unwarranted tra - image character, assists in both image in - log interpretation. The gray well path is jectory changes to be carried out. Within terpretation and minimizing oversteering. calculated from continuous inclination and the portion of the well where the overlays An example of an integrated wellbore the black well path from 90-foot surveys, do not fully match the type log, a combi - visualization in structural geological con - demonstrating improved absolute wellbore nation of real-time azimuthal gamma ray text is shown in Figure 4, demonstrating location with increased data resolution. data, tied in to synthetically modeled how inputting seismic-defined surfaces • (5) Contour maps allow a quick data, permits a high-confidence correlation or synthetic surfaces from nearby wells visual explanation for features occurring to be carried out. The green arrow indicates brings visibility to geological structures as a result of lateral variability according a forward-modeled synthetic gamma ray ahead of the bit. to the well path and its azimuthal deviation for generating a synthetic image ahead from the vertical section azimuth. of the bit using the 3-D model to assist in Fully Integrated Dataset • (6) Curve plots of synthetic and predicting changes and permitting gradual An example of a fully integrated dataset actual gamma ray left and right verify changes to planned trajectories. r of synthetic and actual images with geo - azimuth-related gamma ray and image logical context from a 3-D model is log trends. shown in Figure 5. The integrated data displays seven key elements, as numbered on the figure: • (1) An actual gamma ray (green) curve plotted with a synthetic gamma ray (dashed red) as a primary correlation verification tool. Scaling is adjusted on the synthetic curve at a location of high correlation confidence prior to the lateral portion of the well to account for differ - ences in tool readings between wells. CHRISTOPHER MARK • (2) Actual (below) and synthetic VIENS TOMLINSON (above) gamma ray images. The synthetic Christopher Viens is subsurface in - Mark Tomlinson is technical director image in this example is created using a terpretation subject matter expert at for the Petram Group and the lead soft - wellbore path based on continuous inclination Nabors Drilling Solutions in Houston. ware designer for DynaView™ 3-D data and azimuthal readings extrapolated He joined Nabors in 2011 working on modeling and real-time data interpreta - from surveys. Using a 3-D geomodel allows the development, commercialization tion geological software. With more than correlation tie-ins that update images based and application of MWD/LWD technol - 20 years of industry experience –includ - on full 3-D bed dip change effects. An ex - ogy focused on geosteering, directional ing as a wellsite geologist at Hess Corp., ample of this is indicated by the black drilling and efficient wellbore place - ConocoPhillips, BHP Billiton, Cairn arrow. The actual image comprises a slightly ment. Prior to working with Nabors, Energy, BP, Total and Statoil –he has asymmetrical bedding feature, which is ap - Viens served as a geologist in the Gulf been engaged in operations geology parent on the synthetic image and corre - of Mexico and U.K. North Sea. He holds supporting exploration and development sponds to a topographical feature, as seen a B.S. in geology and earth systems drilling campaigns from the deepwater on the corresponding map location. from the University of Massachusetts, Atlantic to South Texas. Tomlinson holds • (3) Curve plots of synthetic and and is pursuing an M.S. in petroleum a degree in petroleum geology from the actual gamma ray up and down provide geology from the University of Houston. University College of London. a visual correlation check and stratigraphic