59605schD7R1.qxp:59605schD7R1 5/25/07 4:25 PM Page 44 The Expanding Scope of Well Testing Well testing has come a long way since the first drillstem test was run in 1926. From a simple composite packer and valve run on drillstring, the scope of well testing has blossomed into a broad array of sophisticated downhole and surface technologies. Hani Aghar Every E&P company wants to know what type of By measuring in-situ reservoir conditions and In Salah Gas (Joint venture of fluids its well will produce, what flow rates the fluids as they flow from the formation, the testing Sonatrach, BP and Statoil) well will deliver, and how long production can process gives E&P companies access to a variety Hassi-Messaoud, Algeria be sustained. Given the right planning, of dynamic and often unique measurements. technology and implementation, well testing can Depending on the scale of a test, some param - Mark Carie provide many answers to these important eters are measured at multiple points along the New Orleans, Louisiana, USA questions. In one form or another, well testing flow path, allowing engineers to compare Hani Elshahawi has been used to determine reservoir pressures, downhole pressures, temperatures and flow rates Shell International Exploration distance to boundaries, areal extent, fluid against surface measurements of the same and Production properties, permeability, flow rates, drawdown parameters (below). Through well testing, Houston, Texas, USA pressures, formation heterogeneities, vertical operators can extract reservoir fluid samples— layering, production capacity, formation damage, both downhole and at the surface—to observe Jaime Ricardo Gomez productivity index, completion efficiency changes in fluid properties and composition Jawaid Saeedi and more. between the perforation and the wellhead. This Clay Young Houston, Texas Data Measurement Points Bruno Pinguet Surface Acquisition Clamart, France Flowhead Pressure and temperature of tubing and casing Keith Swainson Choke manifold Pressure and temperature Chevron Corporation Heater Pressure and temperature Houston, Texas Separator Pressure and temperature; differential pressure across the gas orifice; flow rates of oil, gas and water; oil shrinkage; basic sediment and water; oil and gas gravity; fluid samples Elie Takla Storage tanks Temperature and shrinkage Hassi-Messaoud, Algeria Subsea test tree Annulus pressure, temperature Bertrand Theuveny Downhole Acquisition Cambridge, England Downhole recording DST pressure and temperature, fluid samples retrieved when test string is brought to surface For help in preparation of this article, thanks to Corey Aures Surface readout Downhole pressure and temperature data retrieved by wireline and David Polson, Houston, Texas; and David Harrison, Sugar Land, Texas. Wireline tools Pressure, temperature, flow rates, samples and various other ArchiTest, CFA (Composition Fluid Analyzer), CHDT (Cased measurements, depending on the suite of tools Hole Dynamics Tester), CleanSep, CleanTest, CQG (Crystal Quartz Gauge), eFire, E-Z Tree, InterACT, IRIS (Intelligent > Data measurement points. Depending on the scale of the test, a variety of Remote Implementation System), LFA (Live Fluid Analyzer measurements may be obtained downhole, at the surface, and at different for MDT tool), MDT (Modular Formation Dynamics Tester), MFE (Multiflow Evaluator tool), OFA (Optical Fluid Analyzer), points along the flowpath. Besides establishing important flow-rate and Oilphase-DBR, PCT (Pressure Controlled Tester), PhaseTester, pressure relationships, the information derived from these measurements PIPESIM, PLT (Production Logging Tool), PowerFlow, helps project engineers track changes in cleanup fluids, understand heat PVT Express, PVT Pro, Quicksilver Probe, SenTREE and flow and hydrate formation conditions in the system and evaluate UNIGAGE are marks of Schlumberger. PhaseWatcher and Vx performance of system components. are joint marks of Schlumberger and Framo. 44 Oilfield Review 59605schD7R1.qxp:59605schD7R1 6/27/07 6:19 PM Page 45 information is vital to predicting the future techniques. This article describes advances Why Test? behavior of a reservoir or well completion. across a range of downhole and surface testing Today, most prospects are explored and then In its most basic form, a well test records equipment. We also discuss the reasons for well produced on the basis of geological and seismic changes in downhole pressure that follow a testing, the strategies applied at different stages data, logging data, and then well testing data. change in flow rate. Often, downhole pressures in the life of a reservoir, and the answers that can Prior to drilling a prospect, seismic data initially and temperatures, surface flow rates and be provided by properly planned, prepared and serve to delineate the depth and breadth of a samples of produced fluids are obtained. executed well tests. Examples from a Middle East potential reservoir. During the drilling process, Variations on this basic theme are carried out gas field and a record-breaking operation in the logging data are used to determine static with regularity. Gulf of Mexico demonstrate the versatility and reservoir parameters such as porosity, lithology, To accommodate different testing needs and high performance provided by today’s well- rock type, saturation, and formation depth, strategies, service companies have developed a testing methods. thickness and dip. Dynamic reservoir properties broad array of innovative testing tools and are measured through well testing. Pressure and Spring 2007 45 59605schD7R1.qxp:59605schD7R1 5/19/07 9:48 PM Page 46 rate perturbations induced by the testing process Well Test Objectives provide important clues to the nature of a Productivity Tests reservoir and its fluids. Obtain and analyze representative samples of produced fluids Wells are tested to determine reservoir Measure reservoir pressure and temperature parameters that cannot be adequately measured Determine inflow performance relationship and deliverability through other techniques, such as mud logging, Evaluate completion efficiency coring, electrical logging and seismic surveys. Characterize well damage Admittedly, in some cases, we can obtain similar Evaluate workover or stimulation treatments measurements through these techniques, but Descriptive Tests the quality or scope may not be sufficient to meet Evaluate reservoir parameters the operator’s objectives. Pressure and tempera - Characterize reservoir heterogeneities ture measurements, flow rates and fluid samples Assess reservoir extent and geometry are keys to understanding and predicting Evaluate hydraulic communication between wells reservoir behavior and production capabilities. Well test data provide inputs for modeling > Well test objectives. The objective determines which type of test will be reservoirs, designing well completions, develop - run, and frequently more than one objective must be achieved. ing field-production strategies and designing production facilities. Well test results are also crucial for reserves estimations. Many countries require flow testing, deliverability and commercial viability of a that bottomhole pressures and temperatures with fluids produced to surface, for reserves to prospect, and is critical for booking reserves. have stabilized. be classified as proven. In addition to estimating Fluid characteristics are particularly important Unlike testing to obtain stabilized bottomhole reserves, these tests provide a means for during the early stages of a prospect’s evaluation, measurements, descriptive tests require transient- directly measuring the aggregate response of when E&P companies need to determine the type pressure measurements. Pressure transients are reservoirs at large scales and for detecting of process equipment they must install to treat induced by step changes in surface production reservoir boundaries. and move produced fluids from the wellbore to rates and can be measured by bottomhole pressure One of the more important reservoir the refinery. sensors or permanent downhole pressure gauges. parameters is permeability. Understanding During development, the operator’s focus The changes in production cause pressure permeability and its directional variability is shifts from assessing deliverability and fluid type perturbations that propagate from the wellbore to essential for developing perforating strategies, to evaluating pressure and flow and ascertaining the surrounding formation. These pressure pulses evaluating fracture or fault connectivity, compartmentalization within the reservoir. This are affected by fluids and geological features predicting well performance and modeling the information is needed to refine the field within the reservoir. While they might travel behavior of the reservoir under primary, development plan and optimize placement of straight through a homogeneous formation, these secondary or tertiary production. Permeability is subsequent wells. pulses may be hindered by low-permeability zones, a scale-sensitive tensorial property; its value During the production phase, well tests are or may vanish entirely when they enter a gas cap. depends on the scale and the direction through conducted to evaluate completion efficiency and By recording wellbore pressure response over which it is measured. And like other reservoir diagnose unexpected changes in production. These time, the operator can obtain a pressure curve that properties, permeability may be heterogeneous. tests assist in determining