From Acid Treatment to Propped Fracturing: Lesson Learned from the Stimulation of an Ultra-Deep HPHT and Tight Carbonate Reservoir

Abstract

Abstract This paper presents 30-years’ detailed case history and technique evolution of an ultra-deep, HPHT and tight carbonate reservoir in eastern China. Productivity after the previous acidizing, pad acid fracturing and multistage acid fracturing is far from satisfactory because of short fracture length and short-term fracture conductivity created in high closure stress and high temperature conditions. The target formation consists of heterogeneous limestone deposits with poor petrophysics, i.e., deeply burial depth (>4800 m), high pressure and high temperature (150°C), poor porosity (3-8%), low permeability (0.04-0.3 mD), high Young's modulus (50-60 GPa), high stress (>90 MPa) and development of natural fractures. Acid-based treatment, including complex pumping of slickwater pads, high- and low-viscosity acids/emulsified acids/cleaning acids/cross-linked acids and various diverting agents, can only bring 3-8 months of appreciable productivity and then rapidly decline to noneconomic production. In this kind of high stress and temperature carbonate formation, high acid leak-off, quick acid-rock reaction rate and inadequate acid-etched fractures are the main restrictions for conventional acid-based technology. To achieve the targets of high fracture complexity, adequate fracture length and long-term fracture conductivity, the following integrated technologies were applied. (1) Propped-frac feasibility analysis was conducted by conventional rock mechanics tests and large-scale rock bulk (75 cm×75 cm×90 cm) hydraulic fracturing physical simulation under in-situ stress, during which the relationship between monitoring events and artificial fracture propagation/morphology, preexisting natural fractures, pumping rate and slurry viscosity were given. (2) The original open-hole completion methods were changed to cementing completion to have fine and precision frac, and higher pumping rate. (3) A new temperature-resistant and variable-viscous fracturing fluid was developed to qualify for pumping treatment. (4) Small size 70/140 quartz sand slugs to erode near-wellbore tortuousness, high strength ceramic proppants to fulfill the main fracture width. (5) Mini-frac test to analyze the stress situation, with the pumping procedures adjusted according to the on-site analysis. The integrated propped fracturing technologies reduced the acid consumption down to zero, which has successfully changed the previous acid-based fracturing mode, significantly reduced the operation cost and improved the fracturing efficiency. This mode has been applied in more than 30 wells in the carbonate reservoir; for example, a total of 880 m3 proppants were pumped into one well (9 layers) at a pumping pressure of 90-95 MPa. Net pressure matching, well testing analysis and microseismic monitoring indicated that longer, deeper penetrated and more complex fractures were created, and the well productivity was greatly improved.