Design and Interpretation of an Interference Test for Determination of Formation Compressibility in a Deepwater Reservoir

Abstract

The design and interpretation of an interference test for determination of formation compressibility in an offshore waterflood is presented. Interference well testing resolves some of the limitations posed by laboratory measurements of formation compressibility, i.e. the sample size is bigger, measurements are made in-situ and the method can be applied reliably in unconsolidated formations. Analytical and numerical PTA (Pressure Transient Analysis) models were used to select the active and observation wells and to design the interference test in terms of lag and lead times associated with the opening and closing of these wells. Some neighboring wells remained on production in between active and observation wells while the interference test was performed due to production constraints. This led to the challenge of unravelling the interference signal from the noise of continued production. Permanent downhole gauge data were utilized in seeking out the interference response at the observation well. Once the interference response was identified, the storativity (porosity-total compressibility product) was determined using the flow capacity (permeability-thickness product) calculated from the pressure and rate history at the active and observation wells. Formation compressibility was then calculated using log derived porosity values. Interference test analysis showed that the formation compressibility was significantly smaller than the pre-test estimates from the lab. The analysis resulted in narrowing of the uncertainty around formation compressibility that was later utilized in field production forecasts. Presence of high frequency high resolution data was critical in deciphering the interference signal. The breakthroughs came by using a range of the calculated flow capacity and total fluid compressibility in determining in-situ formation compressibility, which was in line with an offset analog field that had reliable laboratory measurements. The major impact of this work is in proposing a method for determination of formation compressibility in complex reservoirs and production systems from interference well tests utilizing permanent downhole gauge data.