Fracture treatment design and execution in low-porosity chalk reservoirs

This paper details the results for 33 propped-fracture treatments in low-porosity zones in the South Arne (SA) field, Danish North Sea. To date, seven horizontal wells (2900 in total vertical depth [TVD]) have been completed using 100 tip screenout (TSO) propped-fracture treatments containing 70 million pounds of proppant. The target oil bearing Tor and Ekofisk intervals range from 40 to 120 in of combined thickness, with a Young's modulus and permeability that can vary from less than 0.5 to over 2.5 million psi and 0.1 to 4 in, respectively, along the horizontal section. The wide variations in reservoir and rock properties present significant fracture design and execution challenges. Results indicate that propped-fracture treatments become increasingly more difficult to place as porosity decreases, and this problem is primarily attributed to higher natural fracture/fissure density in the lower-porosity, higher-modulus zones. Production data indicate that these natural fractures or fissures do not measurably contribute to productivity, but can be "activated" under fracturing conditions. Contrary to intuition, pad size and fluid-loss additives must be increased and maximum proppant concentration decreased in low-porosity (low-permeability) zones. In the higher-porosity, higher-permeability northern portion of the field, pad sizes of 35,000 gal containing 20,000 lb of 100-mesh sand allowed the placement of 800,000 lb of proppant at concentrations up to 15 pounds of proppant added per gallon of fluid (ppa). However, in the lower-porosity, lower permeability southern portion of the field, pad sizes of 200,000 gal containing more than 100,000 lb of 100-mesh sand were required to place similar proppant volumes, with concentrations limited to 8 ppa. This paper summarizes field data from 100 treatments, illustrating the design changes necessary to place propped-fracture treatments in low-porosity chalk reservoirs. The paper documents the relationship between chalk porosity, fluid efficiency, and treatment design.