Cyclic Steam Stimulation Effects in Unconsolidated Sandstone Heavy Oil Reservoirs: Application of the Conformal Coring Technique

Summary Jinglou Oilfield is a shallow, thin, extraheavy oil reservoir hosted within an unconsolidated sandstone formation. Following more than 10 cycles of steam injection with declining production performance, the development strategy has shifted toward steamflooding. However, core sampling in this formation poses significant challenges due to the highly unconsolidated nature of Layer IV2, necessitating the development of specialized conformal coring technology to obtain a nondestructive, ‌full-length core sample. This system includes a specially designed coring bit, long nonrotating fiberglass-reinforced plastic (FRP) inner coring barrels, and a functional claw-style core catcher. By conducting full-coring from the upper confining stratum down to the basal mudstone stratum, core integrity was ensured. As a result, excellent core recovery rates were achieved: 96.92% in Well LJ2 and 96.90% in Well LJ3. To prepare samples for analysis, a cryo-conformal wire-cutting technique was developed to extract precise core plugs. Geological core descriptions, along with a range of laboratory tests on both plugs and rock fragments, were performed. The results indicate that the unconsolidated sands of Layer IV2 exhibit excellent reservoir properties, with an average porosity of 35.2% in Wells LJ2 and 36.1% in Wells LJ3 and high average permeability of 1327.7 md and 2261.2 md, respectively. These favorable characteristics, coupled with high oil saturation levels, underscore the layer’s significant potential for thermal recovery. A comparative analysis between the two wells demonstrates that Well LJ3 contains less clay content, exhibits higher porosity, more complex rhythmic layering, and greater heterogeneity. These differences are attributed to grain size redistribution caused by cyclic steam stimulation (CSS), which increased grain diameters and porosity. As a result, the steam flow resistance was reduced, creating favorable conditions for steamflooding implementation. However, steam-affected zones display discernible millimeter-scale flow channels, and ongoing mineral migration further increases reservoir heterogeneity, heightening the risk of steam channeling. This underscores the need for careful control of steam injection parameters, well spacing, and injection-production patterns in future development strategies.