A Method for Evaluation of Formation Damage Due to Fracturing Fluids

Abstract

Abstract The extent of formation damage due to invasion of fracturing fluids during the hydraulic fracturing process was studied. An apparatus, which can simulate reservoir conditions, is described for measuring permeability before and after fluids are pumped across the surface of a core. Dynamic fluid loss was also measured of fluid which passes through the core plug. After the fluid pump schedule was completed, high gas pressure was applied in the reverse direction from fluid leak-off to measure fluid cleanup and the extent of core damage. The system had a wide dynamic range and the measurements have been automated. Introduction A potentially large natural gas resource exists in low permeability reservoirs of the western United States. Massive hydraulic fracturing (MHF) has been used extensively to increase production from tight blanket sands. However, MHF results have been mixed for the Mesaverde and Fort Union sands of the Rocky Mountain area. Various studies have been initiated to develop technology for producing these low permeability, lenticular sands. Many factors can reduce the effectiveness of the hydraulic fracture treatment including damage to the formation and propped fracture from the fracturing fluids, fluid leak-off, clay swelling, precipitation of solids or migration of fines. Some studies have been done which estimate possible formation damage in low permeability reservoirs in order to design fluids or methods to minimize the damage. In connection with the DOE Western Gas Sands sub-program and Multi-Well Experiment, it was desirable to build an apparatus which could evaluate formation damage from fracturing fluids by determining permeability to gas before and after exposure at simulated reservoir temperatures and fracturing pressures. The leakoff rate of fracturing fluids can also be measured with this apparatus. Guar gum and its derivatives, such as hydroxypropyl guar have been effective gelling agents in fracturing fluids for many years. While improved polymers have been developed, guars continue to be the dominate gelling agent in use today. Equipment Description The system was designed to evaluate core damage due to gelled fluid penetration and skin effect at simulated fracturing conditions. Core damage was estimated by measuring gas permeability. In order to simulate fracturing conditions, equipment was developed as shown schematically in Figure 1. The apparatus used a heated, temperature controlled pressure vessel and fluid pump capable of exerting a hydrostatic stress up to 10,000 psi (69 MPa) on a core sample. Test samples were cylinders 3.8 cm diameter and typically 1.2 to 2.0 cm in thickness. Inside the test cell, the sample was mounted in a rubber sleeve between 3.8 cm diameter end pieces. One end piece contained a cavity, 3.3 cm diameter and 0.5 cm deep, to allow fluid flow across the core face. Flow rate across the core surface was typically 2 cm/min. This produced a shear rate of 0.3 to 2 sec-1 over the core surface. Fluids were pumped into and out of this cavity through two holes (0.3175 cm) located for maximum hole spacing in the cavity. Fluid pressure was maintained at the core face by a back pressure regulator located downstream of the fluid cavity as shown in Figure 1. P. 33