This paper describes the general procedure and analysis of an oil well pressure transient test method in which a liquid that is miscible with reservoir oil is injected from the surface at constant rate. Injection significantly reduces surface production test separation and rate measurement equipment requirements, and flaring, thereby reducing testing costs, and safety and environmental risks. Injection of liquid provides for rate control to a precision difficult to attain during production, and although not strictly necessary, allows for a constant rate condition to be practically attained, potentially eliminating a time consuming shut-in and associated test duration and costs. Drilling non-aqueous fluid base oil is a viable miscible injection fluid. A well pressure model is developed that includes the effect of the viscosity difference between injected fluid and reservoir oil. The temperature effect on viscosity is also considered. The model indicates that the duration of the viscosity effect may be short under practical conditions, and is controlled by injection rate. Reservoir parameters estimated from the injection test are identical to those estimated with single-phase production pressure transient tests. A reservoir oil PVT sample must be obtained independently, prior to injection. Conventional pressure transient analysis applications may be used to analyze injection pressure transient data.
This study originated from the desire to obtain reservoir permeability estimates as part of the exploration and delineation of an offshore basin in which the typical play is composed of stacked, fluvial sand reservoirs, forming three-way closures against trapping faults.
A method is desired to assess the productivity of as many of the stacked oil bearing intervals as may be discovered in many or all exploration and delineation wells in the drilling program. The success of future development depends on the exploitation of numerous small, vertically and laterally isolated reservoirs. Evaluation of the productivity of many hydrocarbon intervals allows the estimation of the local distribution of reservoir productivity in various regions of the basin.
Reservoir productivity estimates from production testing are more reliable than that from wireline flow testing and from core analysis, however the disproportionate costs associated with production testing, related to rig time and surface test equipment rental, discourages liberal use of this method. Production testing also carries safety and environmental risks, particularly from flaring. A flow test method that minimizes test duration, and eliminates the need for surface test equipment, is highly desired.
