Fracture Width and Embedment Testing in Soft Reservoir Sandstone

Abstract

Summary A new computer-controlled laboratory technique has been developed to measure propped fracture width and embedment in weakly consolidated cores or unconsolidated sands under simulated downhole conditions. Previously, laboratory studies on cores had determined embedment in hard rocks where embedment was normally limited to 50% of the proppant grain diameter. Several studies also indicated the importance of embedment with one monolayer or less of proppant coverage. The effects of water saturation and fracture-fluid filtrate on formation softness and embedment have not been previously published. Consequently, the objectives of the current paper are to extend previous research results to include soft, weakly consolidated cores and unconsolidated sands with multiple proppant layers. The influence of water saturation and dynamic fluid leakoff on embedment are also shown to be important. The current investigations indicate that embedment becomes a problem when the Brinell hardness (BH) of the formation is less than about 20 kg/mm2 or when the static Young's modulus of formations cores is less than about 2 million psi (13 GPa). Embedment has been determined for cores with static Young's moduli between 0.1 and 1.4 million psi (0.7 to 9.6 GPa). In soft and wet sandstone, embedment can reduce fracture width up to 60% or more for proppant concentrations of 2 lbm/ft2. For unconsolidated sands, embedment is influenced by fracturing-fluid type, water saturation, and downhole conditions. Cyclic loading conditions associated with well shut-ins also increase embedment in unconsolidated sands. This paper reviews and discusses test data on formation cores from south Texas, New Mexico, the North Sea, and the Gulf of Mexico. Most of the commercial fracture-design programs neglect embedment problems in calculating fracture width, while other fracture simulators contain allowances for embedment.