Interpretation of Injection Well Pressure Transient Data in Thermal Oil Recovery

Abstract

Abstract A study was made of methods for burned volume, or steam swept volume, determination from pressure transient test data taken on injection wells. The basis for determination of the swept volume was a new pressure transient model of a composite reservoir. pressure transient model of a composite reservoir. The swept volume was modeled as a radial region adjacent to the injection well, which was considered to have both wellbore storage and a skin effect. The permeability, porosity, and compressibility of the permeability, porosity, and compressibility of the reservoir fluid were considered to be different in the inner region, representing the swept region, and the outer region, representing the bulk of the reservoir. The fluid flowing was considered to be of slight compressibility. The solution technique was analytical, using the Laplace transformation with numerical inversion. This solution has a wide application for problems other than detection of swept volume in thermal oil recovery. Superposition was used to generate the effect of a pressure injectivity test after growth of a swept volume. The results indicate a short duration wellbore storage effect, followed by a semilog straight line whose slope is related to the permeability/thickness of the swept volume. The semilog straight line is then followed by a pseudosteady Cartesian straight line characteristic of the swept volume. Finally, a second semilog straight line appears, characteristic of the permeability-thickness of the unswept region. The permeability-thickness of the unswept region. The discovery of the pseudosteady portion seems to be a major finding. It is possible that this method can be used successfully to detect the swept volume connected with injection wells. Another important finding is that the initial wellbore storage effect dies in a few minutes, and the semilog straight line characteristic of the swept volume occurs almost immediately on shut-in. Further, the pseudosteady period occurs in durations from a fraction of an hour to a few hours. The consequences of this kind of test in thermal recovery operations appear to be extremely important. Early detection of swept volume would mean rapid measurement of the fuel concentration for a combustion operation, and the heat loss from a steam zone! There has not yet been time to study all consequences of this new interpretation method, but it is fair to say that this kind of well testing offers operational information of the greatest importance. Introduction The determination of the swept volume in a thermal oil recovery process is of primary concern. Estimation of the swept volume at intermediate stages of the operation, either in-situ combustion or steam injection, makes the early economic evaluation of the field operations possible. In a recent study, Gates and Ramey show that the fuel concentration of in-situ combustion oil recovery in field operations is an important parameter which can control the economic results of this kind of operation. It is shown that fuel concentration may be determined by a number of methods. One of these is estimation of the volume swept at intermediate stages of the operation. The total fuel consumed may be divided by volume swept to obtain field estimates of the fuel concentration. Another important aspect in thermal recovery concerns the swept volume in steam injection. The volume occupied by steam is a measure of the heat loss from the hot injection zone if the cumulative steam injected is known. In both combustion recovery and steam injection, knowledge of the volume swept by a gas phase contains information critical to the operation. This information has rarely been available, and has usually involved great cost. In field operations, the swept volume has been determined by coring and/or temperature observations made at wells during passage of the displacement front. Attempts have also been made to obtain estimates of the swept volume by means of pressure transient data and tracer test methods. van Poollen utilized the radius of drainage concept to estimate the radius of discontinuity from the transient tests performed on an air-injection well. Kazemi solved performed on an air-injection well. Kazemi solved a pressure falloff test model numerically to calculate the distance to the burning front. In the mathematical description, the effect of temperature on the thermodynamic properties of reservoir fluids was considered by a thermal simulator.