Characterization, Evaluation and Enhanced Oil Recovery in Naturally Fractured Reservoirs

Natural fractures are increasingly recognized as dominant permeability paths in many reservoirs. Unfortunately, there are few guidelines available for geologists and engineers characterizing and engineering naturally fractured reservoirs. This course is intended as an up-to-date summary of an integrated reservoir study including characterization, experimentation and integration of information in determining the most suitable process option in naturally fractured reservoirs. Most of the information originates from the on-going CO2 pilot in the naturally fractured Spraberry Trend Area in West Texas. Information presented from this project in this short course include: core results from several wells including a horizontal core; measurement of fracture populations and spacings from core data; investigation of diagenesis in natural fractures; evaluation of fracture detection logs; detailed study of matrix porosity; evaluation of shaly-sand algorithms for calculation of net pay; measurement of in-situ oil saturation with sponge cores; laboratory measurement of imbibition, capillary pressure and wettability at reservoir conditions, history matching laboratory measurements for up-scaling to reservoir geometry, wettability data for prediction of waterflood performance; reservoir performance analysis during water injection, and laboratory experiments of forced and free-fall gravity drainage with CO2 and use of commercial simulators to match reservoir performance using precisely measured lab and field data.

Two other important case histories’ are also presented. The Midale CO2 flood conducted by Shell Canada is a successful CO2 pilot in a naturally fractured carbonate. A comprehensive data set is presented which clearly demonstrates the role of fracture and matrix characterization, pulse and tracer testing, and laboratory experimentation in development of an adequate history match of waterflooding in order to predict full field performance of the CO2 flood. Geology and fracture characterization is presented along with performance of water injection, effects of rock wettability, gravitational effects, tracer and pulse testing and a complete review of the CO2 pilot is reviewed.

Finally, a case history of a 15 year old gravity stable CO2 pilot in West Texas is reviewed. The Wellman Unit CO2 flood has been one of the more successful CO2 floods on record and the naturally fractured/vugular system of porosity plays an important role in reservoir performance.

We will review reservoir performance in this outstanding project focusing on efficient operation of gravity stable CO2 injection into the top of a carbonate structure