Water Retention Characteristics of Sedimentary Rocks and its Application for Characterization of Unconventional Reservoirs and Well Stimulation Optimization

Abstract

Efficient recovery of oil and gas from unconventional tight and shale reservoirs requires drilling horizontal wells and pumping tens to hundreds of tons of fluid to hydraulically stimulate the wells. Water blended with sand and small amounts of additives is commonly pumped during multistage hydraulic fracturing programs. Immediate flow back of water after fracturing represents only a small fraction of the total water pumped for most stimulated wells. Currently, there is no consensus on whether the pumped water-rock interaction is beneficial or detrimental to hydrocarbon recovery. If the water-rock interaction is proven to be beneficial, slow flow back of fracturing fluid and extended soaking prior to putting a well on production may be preferred. However, field data is often controversial or limited in scope and provides no basis for optimizing field operations to replace the current trial-and-error practice. To better understand water-rock interaction anf hence optimize field operations, we propose to define the water retention/suction characteristics of a rock-water system by directly measuring the water potential (or suction) under varying moisture contents. Water potential or suction force includes capillary pressure, matrix and osmotic potentials and water adsorption, which are controlled by porosity and pore-throat size distribution, mineralogy (especially clays) and fabric, and pore surfaces and their chemo-physical properties. In this study, water potentials of different sedimentary rocks (including Berea sandstone, Pierre shale, Montney, Duvernay, Cardium, and Viking Formations) and the interactions with distilled water and 1 - 5% KCl solutions are measured and water retention characteristics curves are defined. Preliminary results show that the water retention/suction characteristics of these samples differ remarkably from each other, yet relatively similar water-retention characteristics curves are observed for fluids of different salinities interacting with the same sample. The water-retention characteristics are further compared with commonly determined rock properties, including mineralogy from XRD and elemental composition from XRF, pore-throat size distribution from MICP, helium porosity, cation exchange capacity (CEC) and capillary suction time (CST). This study discusses the applications of water potential measurements along with standard core analyses to optimize well stimulation operations.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016