Spontaneous imbibition in asymmetric branch-like throat structures in unconventional reservoirs

Abstract

a b s t r a c tUnconventional reservoirs, especially shale plays, have abundant micro-nano pores. Large volume water was injected into the reservoir during hydraulic fracturing. Fracturing fluid can displace gas in pore spaces spontaneously, called spontaneous imbibition (SI), which has significant effects on the gas production after hydraulic fracturing. Throat structure in matrix is one of the key factors to influence SI process. In this work, at first, complex pore space is simplified as a spatial self-similar branch-like structure composed of several asymmetric sub-throats. Then, within asymmetric branch structure, the imbibition process is calculated by a newly established mathematical model, and the influence of both pore structure and fluid characteristics, including contract angle, viscosity, surface tension, are discussed. Finally, some lab experiments, especially SI tests, are included on shale and sandstone core plugs to verify our model. Results show that the coherence in wettability between the reservoir and the fracturing fluid is important to enhance the microscopic replacement effect. Pore size also has effect influence on imbibition curve. In addition, both in models and experiments, imbibition index n is useful to describe imbibition properties of rocks. In unconventional rock samples like shale, n is less than 0.4, while highpermeability sandstone, n approach to 0.5. These conclusions are useful to investigate the mechanism of spontaneous imbibition and microscopic displacement in unconventional reservoirs.

Experimental Study on Spontaneous Imbibition of CO2-Rich Brine in Tight Oil Reservoirs

“…12 The ability of spontaneous imbibition is critical for the successful development of tight oil reservoirs. 11 It is known that spontaneous imbibition occurs in two types: countercurrent and cocurrent imbibition. 13,14 Capillary pressure is the most dominated power to drive spontaneous imbibition in the fractured tight reservoir.…”

Mathematical Model for the Fluid-Gas Spontaneous Displacement in Nanoscale Porous Media considering the Slippage and Temperature

“…Taking (3), (6), (7), and (10) into (9), the spontaneous imbibition model considering the reservoir temperature and slippage effect can be derived.…”

“…In this process, water will be imbibed into matrix in the fractured reservoir by many influences, including capillary pressure [3][4][5], chemical osmotic pressure [6], pore network [7][8][9], and clay mineral [10], which is called spontaneous imbibition. Also, leakage, lost circulation, and induced fracture in drilling process will lead to fracturing fluid being pumped into formation [11][12][13], which will also cause imbibition process and may change the stress field near wellbore [14] and drilling state [15,16].…”

Visual Laminations Combined with Nuclear Magnetic Resonance to Study the Micro-Unrecovered Oil Distribution and Displacement Behavior of Chemical Flooding in a Complex Conglomerate

“…Recently, primary oil reservoirs worldwide have transitioned into a high water cut period after decades of water flooding. However, on average, 70% of the original oil in place (OOIP) remains with this technique. − A serious shortage of fossil fuels has increased the number of complex reservoirs, such as fractured reservoirs, fractured-caved reservoirs, , and conglomerate reservoirs, and has resulted in the incorporation chemical flooding technologies for the enhanced oil recovery (EOR) purpose. Nevertheless, reports on successful EOR application in conglomerate reservoirs were rarely in past decades.…”