Vertical Interference Test Research Articles

Fluid Migration Characterization of Full-Scale Annulus Cement Sections Using Pressure-Pulse-Decay Measurements

Abstract Fluid migration behind casings is a well integrity problem that can result in sustained casing pressure, undetected leaks to the environment, and potentially very challenging remediation attempts. Understanding the geometric dimensions and extent of annular migration paths is important for diagnosing and effectively treating fluid migration and sustained casing pressure problems in wells. In this study, permeability and micro-annuli sizes in two full-scale cemented annulus test sections are measured using transient pressure-pulse-decay and steady-state seepage measurements. One of the studied sections is a cemented 9 5/8-in. and 13 3/8-in. casing section from a 30 years old Norwegian North Sea production well. A model for predicting the transient pressure decay in annular sections with non-uniform permeability is presented and the permeabilities of the two sections are determined by fitting the transient model to pressure measurements at either side of the test sections and at selected axial positions. For both sections, measured micro-annulus sizes are within the range of effective wellbore permeabilities based on sustained casing pressure records and previous vertical interference tests from other wells. The test sections display measurable axial permeability variations with the bottom part of these vertical sections having the lower permeability. For the retrieved casing section, the axial permeability variation occurs close to the middle of the test section and is attributed to the top-of-cement. Increasing internal casing pressure is found to slightly reduce the equivalent micro-annulus size, indicative of fracture-like response of the migration paths. Using two independent test protocols, we have measured effective permeabilities as well as local permeability variations in full-scale test sections and found consistent results. The study suggests that the transient test procedure can be used to more effectively characterize low-permeable annular cement where it is otherwise time-consuming or difficult to establish steady-state flow conditions.

Assessing the Parameters of the Wellbore Zone from the Results of Vertical Interference Testing of a Gas Stratum

Assessing the Parameters of the Wellbore Zone from the Results of Vertical Interference Testing of a Gas Stratum

Estimation of the Filtration Parameters of a Gas Reservoir by Vertical Interference Testing

A mathematical model of the vertical interference test of a gas reservoir penetrated by an imperfect vertical well is considered. The effect of vertical and horizontal permeabilities and the degree of reservoir penetration on pressure variation curves in the active and responding well intervals is investigated. A method for interpreting the results of vertical interference testing of a gas reservoir based the theory of inverse problems is proposed. It is shown that the results of vertical interference testing of a gas well can be used to estimate the vertical and horizontal permeabilities and porosity of the reservoir in the case of its full penetration.

Pressure transient testing for assessment of wellbore integrity in the IEAGHG Weyburn–Midale CO2 Monitoring and Storage Project

This paper presents an overview of a field program undertaken to assess wellbore integrity in a 54-year old vertical well, conducted as a component of the IEAGHG Weyburn–Midale CO2 Monitoring and Storage Project. The well investigated had been drilled to a depth of 1471m (4824ft), cased with 139.7mm (5.5in.) production casing and cemented with a 1-to-1 blend of Portland cement and pozzolan. Cased-hole logging indicated that the top of the cemented annulus was located approximately 500m (1640ft) above total depth of the well. A built-for-purpose tool was used to conduct pressure transient (vertical interference) testing to assess the hydraulic properties of the cemented annulus within the Watrous Formation, which serves as the regional seal in the vicinity of the Weyburn and Midale fields. A modified version of an existing tool was used to mill through casing then drill small cement core samples. This paper presents an overview of the entire testing program, and a detailed account of the analysis methodology used to estimate the equivalent permeability of the cemented annulus from a vertical interference test. The results of this analysis suggest that vertical permeability of the cemented annulus is very low, falling somewhere in the 7–80 microdarcy range. This result is consistent with cased-hole logging results; more specifically, the combined interpretation of sonic and ultrasonic logs, which suggest effective cement emplacement, and the paucity of casing defects identified by an electromagnetic log, which suggests the cement sheath has effectively shielded the casing from exposure to native formation fluids. The results presented here demonstrate that a well constructed using standard practices and materials of the 1950s can serve as an effective barrier against CO2 migration, although similar work should be undertaken in more wells to confirm that this holds across a broad spectrum of conditions.

Wellbore Permeability Estimates from Vertical Interference Testing of Existing Wells

Wellbore integrity is considered an important risk factor for leakage of CO2 and formation fluids out of geological CO2 storage sites. Quantifying the effective hydraulic parameters that control vertical migration of fluids along the wellbore involves data collection through numerous field and laboratory experiments. The vertical interference test (VIT) is a downhole test designed to measure hydraulic communication of the outside-of-casing wellbore barrier system over a selected well section. Results from these tests can be analyzed numerically to determine the average permeability of the section. Several field surveys of existing wells have resulted in 9 VIT datasets, of which three are presented here. The effective permeability estimates for the three tests span two orders of magnitude, from approximately 1 mD to more than 100 mD. When compared with companion sidewall core analyses of the cement matrix that have permeabilities in the microD range, the VIT data suggest that interfaces or defects in the cement sheath are responsible for flow. Initial analysis of the remaining 6 datasets suggests an even larger range in effective permeability values, as low as microD to more than 1 D, indicating that well permeability can be highly variable from well to well and that high values of permeability are possible. These data provide important insights into realistic wellbore integrity of typical wells in N. America, and help us constrain models for understanding and mitigating risk of leakage during CO2 storage operations.

Pre-injection Baseline Data Collection to Establish Existing Wellbore Leakage Properties

Poor wellbore integrity is a risk in CO2 storage that must be evaluated at any geologic sequestration site. The conditions of five wells from two fields in Wyoming were studied to better understand pre-injection leakage potential in existing wells. Ultrasonic and sonic logging tools mapped the condition of the casing and cement in each well. Permeability testing outside the casing was conducted using two different dynamic testing tools making point and vertical interference test measurements Permeability was also measured through laboratory testing of cased-hole sidewall cores. The results of laboratory measurements were generally in the microdarcy-to-nanodarcy range and indicate that the well cements have not degraded from exposure to the formation brines. The results of vertical interference tests when compared to lab measurements imply interfaces between casing and cement or cement and formation are more significant with respect to leakage than the quality of the cement at the tested location.

Single-well interference slug tests to assess the vertical hydraulic conductivity of unconsolidated aquifers

Summary Meaningful understanding of flow and solute transport in general requires the knowledge of hydraulic conductivity and its anisotropy. Various field methods allow the measurement of the horizontal component (Kh), but vertical hydraulic conductivity (Kv) is rarely measured, for lack of practical field tests. This paper proposes vertical interference slug tests, an adaptation of inter-well interference slug tests to a single well, for the efficient field measurement of Kv. The test is carried out in a single well between a stress and an observation interval that are vertically isolated with a three-packer assembly. An instantaneous pressure pulse is induced in the stress interval and resulting drawdowns are recorded in both the stress and the observation intervals. In a proof-of-concept field study, 12 vertical interference tests were carried out sequentially along a fully-screened well across a moderately heterogeneous and highly anisotropic aquifer made up of littoral silts and sands. A direct-push method was used to install the well, which was completed without sand-pack to allow the natural collapse of sediments in the thin annular space around the screen. Direct-push wells allow the measurement of in situ hydraulic properties of sediments and minimize well construction interferences with hydraulic tests. Drawdowns measured in stress and observation intervals of multiple tests were simultaneously inverted numerically to reconstruct heterogeneous profiles of Kh, hydraulic conductivity anisotropy (Kv/Kh), and specific storage (Ss). Results were validated by comparison of observed versus predicted drawdowns and with field and laboratory measurements of Kh and Kv made along the tested well. Results indicate that the profile of Kv values obtained with vertical interference slug tests follows a similar pattern with depth than the profile with lab measurements made with a permeameter on soil samples collected in the same intervals as the interference tests, which demonstrates that vertical interference slug tests could provide an efficient method for the field measurement of well-scale Kv values.

Analysis of in-situ wellbore integrity data for existing wells with long-term exposure to CO2

Abstract An important aspect of the risk associated with geological carbon dioxide sequestration is the integrity of existing wellbores that penetrate geological layers targeted for CO 2 injection. CO 2 leakage may occur through multiple pathways along a wellbore within the “disturbed zone” surrounding the well casing. The disturbed zone is defined as the annular region along the exterior of the steel wellbore casing that includes the Portland cement sheath, the damage zone of the host rock and the casing-cement-rock interfaces. The effective permeability of this zone is a key parameter of wellbore integrity required for validation of numerical models. Effective permeability may depend on a number of complex factors, including long-term attack by aggressive fluids, poor well completion or actions related to production of fluids through the wellbore. Field tests are essential to understanding the in situ leakage properties of the millions of wells that exist in mature sedimentary basins in North America. We present results from recent field studies of different CO 2 producing wells from both natural CO 2 reservoirs and enhanced oil recovery (EOR) operations. These surveys have included a particular downhole pressure test, the vertical interference test (VIT), designed to determine the extent of hydraulic communication along the exterior of the well casing. The VIT test involves perforating the well casing in two separate intervals, both of which are located within the shale caprock and bracket a zone of cement identified to have a lower quality bond. Once the intervals are isolated with an inflatable packer, the system is pressurized from surface and held at a constant pressure, while simultaneously, the transient pressure response is measured in the lower isolated interval. The pressure transient data is an indicator of the extent of hydraulic communication and is the focus of subsequent analysis. The effective wellbore permeability can be determined through numerical analysis of the VIT data. Our objective is to identify to most effective method of analysis for estimating wellbore permeability. We evaluate two different automated parameter estimation methods, nonlinear regression and shuffled complex evolution metropolis methods. Within this study, we also estimate parameters such as permeability and compressibility of the low permeability shale zone to determine their effect on the resulting estimate of wellbore permeability. The results of this work demonstrate that parameter estimation can be effective at identifying the key parameters associated with wellbore integrity from VIT field tests, and ultimately reducing the uncertainty regarding the integrity of existing wellbores.

Vertical Interference Test in Wells With Downhole Water Sink Completions

Abstract Downhole water sink (DWS) well completions segregate production in the wellbore by producing water from the underlying aquifer and oil—from the oil zone of the reservoir. A pump drains water from the bottom completion to create a pressure drawdown that prevents water from coning up to the top, oil producing zone. Successful application of DWS technology in wells with water-coning problem requires effective isolation between the top and bottom completions of the well. Since DWS technology requires dual completion, the completion is configured for vertical interference testing. The problem is that such test involves flow of two fluids, water and oil. This paper presents a new mathematical model and analysis method for vertical interference testing using top completion (in the oil leg) for production and bottom completion (in the water leg) for observation. The model is analytical and accommodates partial penetration and permeability anisotropy. The analysis method employs a family of type-curves. Examples of possible applications of this new testing method are also shown in this paper.

A New Technique to Determine Horizontal and Vertical Permeabilities from the Time-Delayed Response of a Vertical Interference Test

To determine the permeabilities from a vertical interference test, earlier investigators proposed correlations or type curves based on point source solutions or partially penetrated well solutions. However, it is impossible for these correlations or type curves to cover all possible scenarios in the real world. In more recent years, people use regression analysis to simultaneously match the pressure responses at the source interval and the observation point. With regression analysis, we face the problem of non-unique solutions. Sometimes, estimated permeability could be outside a reasonable range when analyzing noisy data from real tests. In this paper, a new technique is presented to estimate horizontal and vertical permeabilities from the time-delayed response in a wireline vertical interference test. In a vertical test, a pressure drawdown test period is followed by a pressure buildup test period. Because of the delay in response, the pressure at the observation probe continues to drop for some time while the pressure at the source interval is being built up. The maximum pressure drop at the observation probe with regard to the maximum pressure drop in the source interval is time-delayed. Using this time delay and the maximum pressure drop at the observation probe, vertical and horizontal permeabilities can be estimated. A novel numerical scheme is used. The new technique is compared with the previous methods, and it shows its superiority in accuracy. In addition, it can be used in different test configurations. Application issues in real testing conditions are discussed. Finally, two field tests are analyzed using this technique, while an earlier effort to analyze the tests using a conventional method was not successful due to poor data quality.

A New Approach To Evaluate Layer Productivity Before Well Completion

Summary This work presents a new methodology to identify fluid in formation layers and estimate individual reservoir parameters using openhole formation testing techniques. Real-time pressure and fluid identification data are obtained from a new wireline formation testing tool. The tool's dual packer module is needed to isolate individual zones. Field cases from wells located in the San Jorge Gulf basin and in the Neuquen basin illustrate the validity of testing methodology. The reservoir permeability (vertical and horizontal) and formation damage are calculated from pressure transient analysis of buildup and interference pressure data taken from several wells. The results obtained using this technique is consistent with those obtained from testing after well completion (cased hole drillstem test). Evaluation results can be used to decide whether to complete or abandon the tested zone. Fluid type is identified in real time using an optical fluid analyzer. Evaluation of anisotropy on a productive zone scale from the vertical interference test is presented.

Reservoir Description by Interference Testing of the Clayton Field

Summary This paper describes the design, implementation, and analysis of the Claytonfield interference test. The test was performed to gather reservoir informationthat was used with geologic and petrophysical data to assess the magnitude ofinterwell communication and areal and vertical reservoir heterogeneities and todevelop a detailed reservoir description. Interference-test data were evaluatedwith conventional semilog and type-curve-matching techniques. A reservoirsimulator was used to improve the test interpretation, to includerate-variation effects on the observation-well responses, and to develop acomplete reservoir description. The simulator match with interference-test dataused the best geologic and petrophysical information so that the finalreservoir description would honor the best of all available data. Thus, dataidentified by these disciplines were honored during the history match eventhough the match of the test data was insensitive to some of the values. Introduction The Clayton field is located in northeast Michigan in Arenac County (see Ref. 1 for map). Structurally, the field is a large anticline elongated on anorthwest/southeast trend. The northernmost boundary of the field is delineatedby a fault that follows the same trend. The major productive formations in thefield are the Upper and Massive Prairie du Chien. Reservoir property variationsbetween each of these horizons are quite property variations between each ofthese horizons are quite significant. As a result, multilayering is not only apossibility but is expected. The field produces a rich (151 -bbl/MMscf) gascondensate with a dewpoint pressure about 350 psi below discovery pressure. The complex nature of the produced fluid led to a desire to develop areservoir description as early in the life of the field as possible. Evaluationof the extent of both vertical and areal heterogeneity was needed to develop aneconomic depletion plan because, clearly, a depletion plan that includedcycling would be detrimentally affected if high-conductivity layers or regionswere found later. Poor vertical or areal sweep, together with the initialinvestment required for injection of a cycling gas, would preclude anydepletion plan based on pressure maintenance. pressure maintenance. Interference and pulse testing have been used extensively in the petroleum andgroundwater-management industries to determine the extent of communicationbetween producing wells and to derive specific descriptions of areal andvertical property variations that exist in a field. Kamal and Hegemannpresented general overviews of interference- and pulse-testing research done bythe petroleum industry. Review of their studies shows that extensive petroleumindustry. Review of their studies shows that extensive work has been done onthe analysis of interference and pulse-test responses in single-layer systems. The effects of skin, storage, vertical fractures, and areal permeabilityheterogeneity on the interference-test responses of such systems has beenconsidered. The influence of other reservoir heterogeneities (e.g., nearbyfaults, compressible regions, or anisotropic trends) on estimated permeabilityalso has been studied and the resulting theories permeability also has beenstudied and the resulting theories applied to field tests. None of these studies, however, give results applicable to the multilayerreservoir encountered in the Clayton field. Most studies in the petroleumliterature concerned with multilayer systems focus on naturally fracturedformations and/or the concept of vertical interference testing to determine theextent of interlayer communication. Of the four papers in the petroleumliterature that deal directly with the effects of layering on interference andpulse data, two (Refs. 17 and 18) are shown by Onur and Reynolds to be based ontechnically incorrect equations, though the calculations in Ref. 18 arereasonably accurate. Woods and Onur and Reynolds give methods that use combinedrate and pressure data to obtain accurate estimates of layer properties. Without rate data, Onur and Reynolds concluded that a history-matching processwould be the only recourse for data analysis. Kamal advocates this same ideafor multilayer systems. A number of field studies published recently have takenthis history- matching approach, using analytical and/or numerical simulatorsto match the measured data. In this study, we use a multiwell, numerical, gas-reservoir simulator tomatch the measured interference response at six observation wellssimultaneously. Preliminary estimates of average interwell transmissibilitiesare computed with conventional semi- log and/or type-curve-matching methods. Final values of interwell parameters are obtained through trial-and-errormanipulation of parameters are obtained through trial-and-error manipulation ofthese parameters until model and observed pressure responses match. This methodis particularly attractive for this work because one of the major goals was toobtain a preliminary reservoir description that could be used in acompositional simulator to yield a more realistic evaluation of how thereservoir should be depleted. Geologic Setting The Clayton field produces from an Ordovician formation commonly referred toas the Prairie du Chien. JPT p. 524

Vertical-Interference Pressure Testing Across a Low-Permeability Zone With Unsteady Crossflow

Summary Existing analytical models for interpreting vertical-interference pressure tests across the low-permeability zone in a three-layered reservoir usually assume transient radial flow in the high-permeability layers and pseudosteady vertical crossflow in the intervening tight zone. The neglect of vertical flow in the high-permeability layers and of radial flow in the tight layer requires that the permeability contrast between these layers be large. When this assumption is satisfied, however, it is also very likely that the pressure diffusivity in the tight layer is much smaller than that in the high-permeability layers. Thus, to be consistent, the flow across the tight layer must be treated as an unsteady diffusive process driven by the transient pressure changes occurring in the high-permeability layers. In this paper, we extend the analysis of Ehlig-Economides and Ayoub by replacing their pseudosteady crossflow assumption with unsteady crossflow and interpret the crossflow response in terms of near- and far-boundary flow-response functions for the tight zone. The inclusion of unsteady crossflow requires one additional dimensionless parameter and results in a set of governing equations that are different from but similar to the equations of the dual-porosity, dual-permeability model with unsteady interporosity flow presented by Wijesinghe and Culham for naturally fractured reservoirs. As in the case of dual-porosity, dualpermeability reservoirs, the pressure response with unsteady crossflow is significantly different at early times from that for pseudosteady crossflow under practical conditions. Exact and asymptotic solutions and type curves for the wellbore pressures at the upper and lower perforations are presented for flow-rate-specified vertical interference tests. The solutions account for different formation properties, wellbore storage, and skin factors of the two high-permeability layers and the formation properties of the tight zone. The impact of unsteady and pseudosteady crossflow model assumptions on the wellbore pressures and on the spatial distributions of fluid pressures and crossflow rates is examined in detail.

Vertical Interference Testing Across a Low-Permeability Zone

Summary Bremer et al.1 described a mathematical model for analysis of a vertical interference test across a low-permeability zone. The test configuration called for recording wellbore pressure vs. time opposite one permeable layer while fluid is produced from the second permeable layer. The two permeable layers are separated by a tight zone of low vertical permeability. This paper offers a more general interpretation technique that allows for distinct properties in the two permeable layers and includes wellbore-storage and skin effects. The purpose of the vertical interference test across a low-permeability zone is to determine the vertical permeability of the tight zone separating the two permeable strata. The results of such a test may influence well completion decisions for primary, secondary, and enhanced oil recovery. Of particular interest is the estimation of the fluid flow rate into or out of the packed-off layer. Field test data have been analyzed with the new techniques. By including wellbore-storage and skin effects, the model presented in this study provides a much better fit with the data presented by Bremer et al.1 Because this method allows for determination of formation properties in both permeable zones, simultaneous recording of the pressures in both the producing and packed-off layers is recommended. Results indicate that the wellbore- pressure difference between the flowing and packed-off zones becomes constant after sufficient time and depends only on the permeabilities in the three zones. If wellbore-storage effects can be minimized, the behavior of pressure vs. log time may exhibit two distinct slopes: the first providing for calculation of the permeability in the flowing layer, and the second allowing calculation of the permeability in the packed-off layer. Type curves are presented for analysis of both the pressure and pressure-derivative transients. The interpretation method is suitable for broader application than previous methods would allow. An important result is an estimate of the rate of crossflow between layers separated by a low-permeability zone.

Analytical Model for Vertical Interference Tests Across Low-Permeability Zones

Abstract A mathematical model is developed that describes fluid flow and pressure behavior in a reservoir consisting of two permeable zones separated by a zone of low permeability, or a "tight zone." This model can be used to design and to interpret buildup, vertical, interference, and pulse tests conducted in a single well or multiple wells across lithological strata. Dimensionless pressure functions and corresponding parametric type curves are derived to interpret vertical interference test data for tight-zone vertical permeability. Application of these type curves is illustrated using field data from two vertical interference tests. Test results obtained with the tight-zone model are shown to compare favorably with results obtained by using (1) computer simulations and (2) Burns' method based on the uniform anisotropy assumption. Computer simulation using a numerical model also shows that high near-wellbore conductivity from a packer leak or poor cement job could not have adversely affected test results. The model presented and the type-curve interpretation method outlined are accurate for designing and interpreting single-well vertical interference tests across low-permeability zones.

Field Application of Vertical Well Testing Methods With a Case History

Summary This paper presents information on a field application of vertical well testing techniques and data analyses and summarizes the results of a case history study on vertical pulse testing. The testing program revealed that it is possible to operate and analyze vertical well tests. A modified Prats technique proved adequate for analyzing vertical pulse test data. Introduction Vertical well testing is a type of interference test conducted between two perforations in the same wellbore. The technique, which includes both vertical interference and vertical pulse testing, provides a way to estimate formation vertical and horizontal permeability. Knowledge of those properties can be used to evaluate the extent of vertical communication in a reservoir near a tested well. Such information can be useful in a selective flooding or vertical conformance control program.This paper presents information on a field evaluation of vertical well testing techniques and data analysis, and summarizes the results of a case history study of vertical pulse testing. A companion paper describes vertical well testing techniques, analysis, and design. Vertical pulse testing in Marathon Oil Co.'s Oregon Basin field in Wyoming provided the basis for information presented in this paper. Four vertical pulse tests were conducted in and between Tensleep reservoir zones at Baston A Well 13 in that field. One goat in the program was to evaluate vertical well testing and data analysis techniques. A second goal was to determine the extent of vertical communication in the Tensleep reservoir. The case history study reports results of the field evaluation.On the basis of experience gained, we conclude that one can operate and evaluate vertical pulse tests in most situations. The main difficulty in testing at Baston A Well 13 was the wide difference between sandface and surface injection rates caused by low reservoir pressure and high wellbore storage. That situation created some analysis problems that we were able to overcome for the most part. By appropriate modifications of field procedures, future tests should provide adequate data.We were not too successful in achieving the second goal in the testing program. Analysis results for the first three tests conducted showed an abnormally high vertical permeability. This anomaly - the result of flow behind the casing - caused problems in evaluating vertical permeability in the Tensleep reservoir. However, we were able to estimate vertical permeability using test results plus computer simulation studies. There was no response during the fourth test. That indicates no communication between the tested zones, either through the casing cement or in the reservoir.The testing and analysis procedures used in the four vertical pulse tests were basically the same. With the exception of Test 4, all tests show the same type of response behavior. For this reason, and for the sake of brevity, only Test 1 is discussed in detail in this paper. However, analysis results are presented for all four tests. Vertical Pulse Tests Run in Oregon Basin Field The well used for vertical pulse testing is located on the south flank of the south dome of Oregon Basin field in Wyoming. Testing was done after drilling, logging, and casing the well but before final completion. JPT P. 1113^

Analysis and Design Methods for Vertical Well Testing

Two methods, each modifications of a technique proposed by Prats, are recommended for analyzing vertical well tests for horizontal and vertical permeability. The methods are derived and presented, and their permeability. The methods are derived and presented, and their applicability is demonstrated using simulated vertical pulse test data. Information is provided on the behavior of vertical pulse tests in one- and two-layer reservoir systems. Vertical Well Testing Provides Important Reservoir Data Vertical well testing between two perforations in the same wellbore, including interference and pulse testing, provides a mechanism for estimating both horizontal and vertical permeabilities near the well. Those permeabilities are important in reservoir analysis because they can indicate if there is communication between zones or behind the pipe of if tested zones are separated effectively. Such knowledge is essential for selective flooding and vertical conformance control.This paper presents information about vertical well testing techniques, analysis, and design. Both interference and pulse tests are discussed. A future paper will describe field evaluation of the paper will describe field evaluation of the techniques, with a case history.Vertical well testing is accomplished by isolating the upper and lower portions of the wellbore and using one portion as a pressure observation location and the other as an active (injection or production) location. Either production or injection production) location. Either production or injection can be used at the active perforation, but an injection-type test usually is easier to run. All vertical well tests are a class of interference test because pressure is measured at a location other than the injection or production location. A vertical well test is a special type of vertical interference testing that uses alternating (usually equal) periods of injection (or production) and shut-in repeated for several cycles. Because of its periodic nature and the resulting ease in periodic nature and the resulting ease in recognizing test response, pulse testing generally is the preferred technique.Two methods have been devised for analyzing vertical pulse and interference tests. The methods are based on the same flow equation and give comparable results, although the mechanics of using them are somewhat different. Both rely on computer programs that normally would be used for single-well transient tests or multiwell interference tests. The two methods were tested using simulated test data and gave reasonable estimates of formation properties. This paper explains the methods, provides theory for the methods, and illustrates their applicability in analyses of test cases.The only method in the literature for analysis of vertical pulse tests without a computer (proposed by Falade and Brigham) is limited to situations with negligible wellbore storage. In such cases it appears to work adequately.The vertical well test analysis methods given her can be applied when there is significant wellbore storage at both the pulsing and observation perforations. That situation specifically is excluded perforations. That situation specifically is excluded from most published analysis methods. P. 505

New Single-Well Test for Determining Vertical Permeability

A short-term vertical interference test between isolated sets of perforations in the same well can be used to determine the vertical perforations in the same well can be used to determine the vertical and horizontal permeabilities of each tested interval as well as to determine the effectiveness of tight streaks as barriers to vertical flow. Introduction Vertical flow is an important mechanism in many petroleum reservoirs. Yet no adequate method has petroleum reservoirs. Yet no adequate method has heretofore been proposed for determining the in-situ vertical permeability in a reasonable length of time. Core measurements of vertical permeability do not necessarily represent effective in-situ values. Cores may be representative of only localized pockets of high or low permeability, and total reliance on cores night cause one to overlook the presence of vertical fractures altogether. Conventional single-well tests, interference tests, and pulse tests commonly are used to determine horizontal transmissibility, k(h)h/mu, but generally are not designed to measure vertical transmissibility or other groups containing vertical permeability, k(v). permeability, k(v). A new well test described here provides a measure of in-situ vertical permeability as well as the previously measurable horizontal permeability. This previously measurable horizontal permeability. This "vertical test" determines the vertical diffusivity, k(v)/phi c mu, and horizontal transmissibility, k(h)d(1)/mu (where d(1) is the length of injection or production perforations). The test is basically a production perforations). The test is basically a short-term interference or pulse test normally lasting less than a day. Conventional down-hole equipment is used between two isolated sets of perforations in the same well, as shown in Fig. 1. A "layer-cake" description of a reservoir can be obtained by running two or more vertical tests in the same well, each over different vertical intervals. Each test can be analyzed for the effective vertical and horizontal properties of the interval tested. In addition, a vertical well test can determine the effectiveness of a shale barrier or tight zone by testing across it. Field use to date of vertical testing has confirmed its utility in reservoir description. Mathematical Basis Interpretation of vertical tests for k(v) and k(h) is based on comparison of measured pressure response with computer-generated response. Pressure behavior during a vertical test in a homogeneous, anisotropic, infinite-acting reservoir (infinite in radial extent and bounded in both directions vertically) is given by the diffusion equation for slightly compressible fluids with negligible gravity effects.,.........(1) with the boundary and initial conditions = 0, ..........(2) lim,.....(3) lim, ,....(4) ,........................(5) ,........................(6) JPT P. 743