Application of Injection Tests for Reservoir Appraisals: A Conceptual Study

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.

Green Methodologies to Test Hydrocarbon Reservoirs

“…Although an injection/fall-off test is similar to a conventional drawdown/build-up test, a distinction between the two is necessary when the properties of the injected and reservoir fluids are different (Gan et al, 2002). In fact, the analytical models adopted to interpret conventional or not conventional production tests have been developed under the hypothesis of mono-phase flow in the formation.…”

“…A thorough discussion on the applicability of analytical interpretation models when different fluids are interacting in the reservoir can be found in the technical literature (Beretta et al, 2007;Gan et al, 2002;Abbaszadeh and Kamal, 1987;Bourdet, 2002). If the transition zone is neglected two zones with different mobilities and compressibilities are left.…”

An Effective Criterion to Prevent Injection Test Numerical Simulation from Spurious Oscillations

“…An injection test consists substantially in injecting a fluid, typically brine, in a potential pay zone and monitor the pressure response during the injection period and the subsequent fall-off period, in which the well is closed and the pressure tends to return to the equilibrium value. Although an injection/fall-off test is similar to a conventional drawdown/build-up test, a distinction between the two is necessary when the properties of the injected and reservoir fluids are different (Gunawan et al, 2002). In fact, the physics of injection tests is characterized by the presence of two phases in the reservoir, the hydrocarbons originally in place and the injected fluid; therefore, fluid saturations change dynamically during injection in both space and time and the permeability of the reservoir rock to each fluid will be dependent on saturations.…”