Hydraulic fracturing formations at temperatures >375°F is challenging because of the unavailability of suitable fluids with sufficient thermal stability. Applying low thermal-stability fluid for fracturing these formations causes rapid viscosity degradation, leading to high fluid loss and narrow fractures. This paper discusses strategies adopted in successful offshore high-pressure/high-temperature (HP/HT) formation multistage fracturing using lower gel-loading fluid.
Commercially available gelled fracturing fluids are made using polysaccharides or their derivatives. Glycosidic linkages in polysaccharides begin breaking rapidly at temperatures >375°F, making polysaccharide fluids unsuitable for HP/HT hydraulic fracturing when significant fracture conductivity must be achieved. Strategies were planned to cool down the reservoir and bring the temperature into the polysaccharide fluid thermal-stability range. The reservoir's temperature is approximately 400°F and has a high clay content. Performing fluid/clay mineralogy interaction tests optimized the type and concentration of the clay control additive; performing stability tests on an HP/HT viscometer optimized the crosslinked fluid formulation with low polysaccharide gelling agent concentration. Considering well completion and reservoir complexity, a limited entry technique was used to achieve maximum zonal coverage.
The polymeric clay control additive performed better than conventional salt in clay inhibition; it did not have a negative effect on fluid thermal stability. Metal-crosslinked fracturing fluid containing 45-lbm/1,000-gal gelling agent was stable for approximately 1.5 hours at 350°F; results were comparable with gel having 50-lbm/1,000-gal gelling agent. The strong metal-crosslink bond provided higher thermal stability to the gel.
Based on petrophysical log analysis, six zones were identified for hydraulic fracturing treatment; however, during execution two zones were combined, so five treatments were successfully executed on a well with a bottomhole static temperature of 400°F in the Krishna Godavari Basin (eastern coast of India). A total of 1,000,000 lbm of high-strength proppant was placed into the formation using approximately 17,000 bbl of crosslinked gel. The zone was perforated in individual clusters; a limited entry technique was used to help treat the zone. The limited number of perforations resulted in increased pressure differential across perforation clusters, leading to effective diversion and maximum zonal coverage. Each stage was isolated using a bridge plug, and diagnostic pumping was conducted before each main treatment. To gain insight into the actual geometries achieved, data acquired during the treatment were history matched using a full 3D grid-oriented hydraulic fracture simulator and compared to initial simulations.
Fracturing in extreme high temperature with high pressure poses significant challenges. This paper details successful hydraulic fracturing applications in one such reservoir regarding optimization and utilization of a fluid system to achieve optimum fracture geometries and minimizing formation damage and discusses other aspects of fracturing this reservoir, such as perforation strategies and operational tactics for successful placement of multistage hydraulic fracture treatments.