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Abstract In this work we present a new theoretical understanding of pressure data obtained at a water injection well. The theory provides new physical insight on how permeability heterogeneity, saturation gradients and mechanical skin factor combine to influence the pressure response at the well. Based on the theoretical equations, methods for analyzing injection/falloff pressure data in both homogeneous and radially heterogeneous reservoirs are presented. For homogeneous reservoirs, we present procedures for estimating the mechanical skin factor from injection or falloff pressure data. Our theory provides a procedure for analyzing the pressure response during the second injection period of a two-rate test. It is shown that the information that can be obtained from a two-rate test is similar to that obtained from a falloff test. Introduction Injection testing is pressure transient testing during injection of a fluid into a well. It is analogous to drawdown testing for both constant and variable rates. Shutting in an injection well results in a pressure falloff which is similar to pressure buildup in a production well. However, the distinction between injection/falloff and conventional drawdown/buildup testing is that the flow characteristics of the injected fluid are different from those of the original reservoir fluids so that multiphase reservoir flow has to be considered in order to understand these tests. A novel insight into the theory of multiphase flow pressure transient testing was presented by Thompson and Reynolds. Their theory describes the averaging process that occurs during multiphase flow drawdown and buildup and explains pressure transient behavior of both single and multiphase flow in radially heterogeneous reservoirs. Although the focus was on gas condensate reservoirs, their studies included injection/falloff testing. In summary, the theory stated that well test mobilities reflect weighted average mobilities in those regions of the reservoir where rate is changing with time and where mobility is changing with time, i.e., (1) where, C1 is a units conversion constant, (see Nomenclature), KR is a "rate kernel", defined as, and KM is a "mobility kernel" defined as. In the case of injection/falloff testing, they argued that if changing mobility were the dominant factor occurring during the injection phase, it could explain the common belief that it is impossible to see beyond the "flood front" during injection. However, further numerical experiments on injection testing indicated that there were some cases where permeability beyond the flood front could affect injection-well pressure data. In this work, we investigate this apparently anomalous behavior carefully, and show that for injection/falloff testing, multiphase flow pressure derivative data yield information both about the reservoir region close to the moving flood front and the unflooded zone ahead of the front. Approximate Analytical Injection Solution In this section, we derive an approximate analytical solution for water injection in a radially heterogeneous reservoir. We assume an infinite cylindrical reservoir with a fully penetrating injection well of radius rw at the center of the reservoir. Wellbore storage effects are neglected. Water is injected into the reservoir at a constant rate, qwBw RB/D. Except for connate water, the reservoir is initially assumed to be filled with oil of a small and constant compressibility. The reservoir is made up of N+1 concentric cylinders having radii of r1, r2, … rN, with corresponding permeability values of k1, k2,…,kN, kN+1 respectively. P. 67^
To achieve optimal production from unconventional reservoirs, it is useful to determine the permeability, pore pressure, and state of stress of rock strata. Doing so will lead to properly designed treatments, realistic predictions of well performance, and a basis for normalizing reservoir contribution when evaluating completion and stimulation effectiveness.
An effective way to derive the necessary reservoir information is to conduct in-situ pressure transient tests. Since it is difficult to inject fluid into or withdraw fluid from the pore network of tight rock, diagnostic fracture injection tests (DFIT) have been employed to create an analyzable pressure decline response, as well as to derive the minimum horizontal stress via fracture closure identification.
This paper is a study of numerous DFITs conducted in unconventional reservoirs throughout the world to evaluate the reservoir and geomechanical characteristics of the pay zone and bounding intervals. Within this body of work, experiments were implemented to study the impact of testing methods on the test response and various types of analysis methods documented in the literature were implemented and compared. The paper summarizes findings and introduces tactics for planning/conducting tests and evaluating results in a variety of unconventional reservoir types.
Abstract This paper presents a conceptual study to evaluate the effectiveness of injection tests for the collection of dynamic reservoir data during project appraisals. A 2-D xy numerical model was built using representative field data to simulate the process and pressure response during injection and falloff tests for various boundary conditions. Various injection cases were studied: water injection into oil and gas condensate reservoirs, nitrogen injection into oil reservoirs, and nitrogen injection into dry gas, lean retrograde gas condensate, and heavy retrograde gas condensate reservoirs. For the case of nitrogen injection, we used a compositional option to model the phase interactions during the tests. Simulated pressure responses were analyzed using single-phase analytical solutions provided by a commercial well testing software package. We found that a composite reservoir model provided by well testing software package can be used to analyze the injection and falloff tests data. Permeability and distance to the boundary can be estimated for most of mobility ratios. Permeability found during the injection test is the effective permeability to the injected fluid. For the falloff test, it is possible to obtain the effective permeability of reservoir fluid. Type of boundaries can be detected and the distance from the well to the boundaries can be estimated by using effective system compressibility. The ability of boundary detection and mobility calculations are heavily dependent upon the optimal injected volume, and therefore, rely on proper well test design. For test design, the optimum injection volume is very important since it affects the time required to have radial flow in the uninvaded zone (‘reservoir zone’) which is necessary for the interpretation of reservoir boundary. Mobility ratio affects the length of transition zone and also the interpretation of reservoir boundary. Therefore, test design using a reservoir simulator is critical for determining the injection volume and the phase interactions if any, and understanding the pressure response. In addition to the above results, we utilize the material balance approach to calculate the compartment volume thus enhance the interpretation of injection and falloff tests. The results of this study provide a new and comprehensive overview of the practical application of the injection and falloff tests for the collection of dynamic reservoir data during project appraisal - with zero flaring. Introduction Injection testing is pressure transient testing during injection of a fluid into a well. It is analogous to drawdown testing for both constant and variable rates. Shutting in an injection well results in a pressure falloff that is similar to pressure buildup in a production well. Although an injection/falloff test is similar to a conventional drawdown/buildup test, the distinction between the two is necessary when the properties of the injected and formation fluids are different. Typically, injection/falloff tests are used for reservoir management of water flooding and enhance oil recovery projects, which usually happen later in the life of a field following primary depletion. Now, we are considering the use of injection/falloff tests during initial development planning of a field where these tests would be performed on appraisal wells drilled prior to the decision to develop the field.
A step-rate injectivity testing program was designed to obtain additional operating parameters and reservoir data of the Roosevelt Known Geothermal Resource Area (KGRA), Beaver County, Utah. The testing program consisted of injecting fresh water at rates varying between one half (0.5) BPM and sixty (60) BPM into four producers and one injector. The rates, wellhead pressures and subsurface pressure data collected was used to size injection pressure data collected was used to size injection equipment for future field development, verify the highly fractured reservoir and cross check previous permeability calculations by using the multiple rate permeability calculations by using the multiple rate transient technique.
In order to obtain the required permits for continued field development of the Roosevelt KGRA by construction of a 20 MW (net) power plant, a Plan of Development (POD) was required by the United States Geological Survey (USGS). The step-rate injectivity testing program was designed to obtain injection rates and pressures within the field boundaries for sizing injection equipment for the Plan of Development and collecting additional data on reservoir properties. Figure 1 is a map of the Roosevelt field properties. Figure 1 is a map of the Roosevelt field showing the locations of the five (5) wells tested. Normally step-rate testing is limited to injection wells for the purpose of determining formation parting pressure, and the injection rates are below parting pressure, and the injection rates are below 5 BPM with injection time for each step either thirty (30) or sixty (60) minutes. The Roosevelt KGRA is a highly fractured water dominate reservoir and in order to obtain measurable pressures during injection, high rates were required. The lack of availability of high volumes of fresh water necessitates the short pumping times for each step.
APPLICATION OF EQUIPMENT AND PROCESSES
The initial testing procedure called for injection rates of one half (0.5) BPM and proceeding in steps of one, two, three, four and five barrels with time increments of ten minutes between each rate step. Once the five BPM had been pumped for ten minutes, the rate was to be increased to twenty (20) BPM with rate increases in ten (10) barrel increments from twenty (20) BPM to sixty (60) BPM with a pumping time of five (5) minutes for each pump rate. During each rate-step, the end point pressure would be monitored and recorded. It was anticipated that a maximum of sixty (60) BPM would be reached, and once this rate had been pumped for five minutes, the test would be terminated. The low rates of one half (0.5) BPM to five (5) BPM were used in an effort to insure that the casing was cooled to the casing shoe as all wells tested were open hole completions.
The equipment used to obtain the high injection rates
1) Five, one thousand (1,000) hydraulic horsepower (HHP) frac trucks.
2) A blender to super charge the water to the frac trucks.
3) Two, (500) barrel frac tanks to store the fresh water.
4) Two, 100 barrel water trucks connected to the frac tanks.
5) Twelve hundred (1200) barrels of fresh water
6) Heise pressure gauges to measure surface pressure. pressure. 7) Subsurface pressure chambers connected to a Heise gauge by 3/32" stainless steel tubing filled with Nitrogen (N ) were suspended in wells No. 52-33, 25-15 and 13-10, to measure subsurface pressure.
This paper presents the results of a review of over eight hundred historical pressure tests conducted in the McElmo Creek Unit of southeast Utah. Three hundred pressure transient analyses were performed. The paper describes how this data base was used to characterize trends in pressure, reservoir quality and stimulation effectiveness. The various techniques used to acquire the pressure data are also discussed.
Reservoir pressure testing is conducted throughout the producing life of most oil and gas fields. The testing history for McElmo Creek is shown in Fig. 1. The periods of high test frequency correspond to important stages in the field's development. Initially, pressure data was acquired to monitor primary depletion prior to waterflooding. When waterflooding was initiated in 1962, tests were conducted to assess the pressure support from the water injection. In the early and mid 1980s, pressure data was acquired in advance of the carbon dioxide (CO,) miscible flood (initiated in 1985) to ensure that the reservoir pressure was in excess of the minimum miscibility pressure. By the 1980s, it was recognized that pressure transient testing is valuable to reservoir characterization, and as a result, more pressure transient tests (as opposed to static pressure tests) were conducted.
The database of McElmo Creek pressure test data includes some eight hundred and fifty individual tests. Approximately three hundred of these tests include a complete pressure transient suitable for analysis. The large number of tests permits comparison of parameters both within a single well over time and between surrounding waits. This paper provides details on the testing procedures and describes how the test data was used to characterize trends in pressure, reservoir quality and simulation effectiveness.
The McElmo Creek Unit, operated by Mobil in the Aneth field of San Juan County, Utah, includes 312 wells of which 119 are injectors. The Unit has been waterflooded since 1962 and has been under a partial CO2 flood since 1985. This field produces oil from the Desert Creek and Lower Ismay zones of the Paradox formation, which is of Pennsylvanian Age. A typical log, showing porosity and core permeability, is presented in Fig. 2. Detailed geologic and fluid property data are available in the literature.
PRESSURE DATA ACQUISITION
Most of the pressure transient data was acquired through injection falloff tests. Approximately half of these pressures were recorded using downhole gauges. The other half of the falloff tests recorded the tubing pressure at the wellhead and relied upon a pressure extrapolation to get bottomhole pressure. Approximately one-quarter of the pressure transient data was acquired through pressure buildup testing on producing wells. Most of these tests were performed early in the field's life when wells were capable of flowing. Only a few drawdown tests were ever attempted.
In general, analysis of the pressure transient data for McElmo Creek wells is straightforward. Those tests which cannot be analyzed adequately with a homogeneous, radial flow model can usually be modeled with a high conductivity fracture model. None of the tests analyzed show the characteristics of a dual porosity (naturally fractured) reservoir. All injection at McElmo Creek is carried out below the formation parting pressure. Dynamic fracture characteristics are therefore not a concern with pressure transient analysis.
CONVENTIONAL PRESSURE TRANSIENT TESTING
Most of the pressure tests run at McElmo Creek, whether simple static pressure tests or full pressure transient tests, rely on the use of downhole pressure devices. These devices have the benefit of accurately measuring true bottomhole pressure without the need to extrapolate pressure from the surface.