A Shale Matrix Imbibition Model – Interplay between Capillary Pressure and Osmotic Pressure

Abstract

High and efficient deliverability of stimulated reservoir volume through a hydraulic fracturing treatment relies on three segments: fluid flow from matrix to the interface between fracture and matrix media, fluid-rock interaction at the fracture-matrix interface, and conductivity of fracture network. Thus, fluids and salt exchange between matrix and fracture network are critical and worth detailed investigation. Moreover, matrix imbibition as an important EOR mechanism has been extensively studied but the focus was mostly given to capillary effect. However, for shale, due to the pore structure and clay content, some physico- or electro-chemical forces at molecular level cannot be overlooked anymore, such as osmosis. A multi-mechanistic numerical shale matrix imbibition model is developed. The model takes into account dynamic water movement caused by capillary pressure and osmotic pressure as a function of water saturation and salt concentration, respectively. The rock matrix is considered as the mixture of two different components, one with small nano/micro-pores and semi-permeable membrane property and the other having larger meso-pores. The model properly simulates water and salt transportation occurring across the matrix-fracture network contact surface driven by capillarity, osmosis, and salt diffusion. To honor the physics, the salt/ions concentration equation differs from previous work by removing the osmosis component and a new membrane efficiency coefficient is defined and properly incorporated in the model. Spontaneous imbibition test results were used for matching and validation purposes. The simulation results well explained laboratory high-salinity water imbibition curve, which can be divided into three processes. Initially, a capillary driven imbibition sucks high salt-concentration water into matrix near the matrix-fracture contact surface. However, due to the significant salinity contrast between imbibed fluids and in-situ matrix salinity, a drainage process can be induced. Eventually, as salinity difference decreases and osmosis is weakening, final imbibition stage starts. This model provides a basis for laboratory measurements interpretation and brings some insights to reveal the underlying mechanisms for field post-frac flow-back behavior.

Impact of Salinity and Mineralogy on Slick Water Spontaneous Imbibition and Formation Strength in Shale

“…The incorporation of both mechanisms is critical in properly modeling water dynamics in both fracture ,− and matrix imbibition . Li et al integrate the two mechanisms to develop a one-dimensional imbibition model for shale using samples from the Horn River basin …”

A Critical Review of Osmosis-Associated Imbibition in Unconventional Formations

“…Neuzil and Provost [89] observed the anomalous fluid pressure in a subsurface when they performed osmosis measurements on moderately compacted high clay content Pierre shale. This can be illustrated by electric double layer (EDL) theory [90]. Clay particles are naturally and commonly negatively charged.…”

Efficiency of enhanced oil recovery by injection of low-salinity water in barium-containing carbonate reservoirs

“…Measurements were repeated three times for each experiment, at room temperature. For measurement of the contact angle in the porous rock, the disks were first polished with six different grades of sandpaper to minimize the roughness effect (Al-Shaikh and Mahadevan 2016; Li et al 2016). The standard deviation of the measurement was in the range of 0.13-0.21, which indicates that the measured contact angles were relatively consistent.…”