MINIMIZING THE LOST CIRCULATION IN OIL-BASED DRILLING FLUIDS USING OIL ABSORBING EXPANSIVE RUBBER

Fast dissolving polyester oil-soluble polymers seriously compromised the rheological properties as well as the plugging efficiency of oil-based drilling fluids (OBM). In this study, an oil-soluble expansive copolymer (EPDM-LCM) constituted of ethylene propylene diene monomer (EPDM) grafted with 4-tert-butyl styrene (t-BS) in the presence of cross-linking divinylbenzene (DVB) was prepared by free-radical copolymerization in order to reinforce the hydrophobic property which is beneficial to reduce the dissolution rate of copolymer and the molecular structure was characterized by Fourier transform infrared spectroscopy (FT-IR). Test the thermal stability of EPDM-LCM by differential scanning calorimeter (DSC). The oil-expansive rate of EPDM-LCM examined with respect to time and temperature was compared with other conventional plugging agents. EPDM-LCM showed lower solubility in oil at room temperature (25oC) but the expansive rate reached 210% after 150 min at high temperature (180°C) as to indicate not only its relatively little impact on the rheological properties of drilling fluid before invasion into high-temperature formation zones, but also to boost the expansivity of plugging materials in the formation. In combination with EPDM-LCM, bridging material (calcium carbonate), filling material (graphite), and tensile materials (fiber) were exploited to develop a composite plugging agent suitable for different cracks. The formulation of composite plugging agent for micro-cracks obtained as follows: 1% EPDM-LCM+1% CaC03 (600 mesh)+0.5% GP (30-50 um) could achieved a maximum pressure bearing capacity of 7.75 MPa while that of macro-cracks prepared as follows: 1% EPDM-LCM +1% CaCO3 (600 mesh)+0.5% GP (35-65 um)+1.5%fi-ber (3-5 cm) was 7.0 MPa. Both composites demonstrated good plugging performance. In addition, the formulated composite plugging agents had relatively little impact on the rheological properties of OBM. This strongly translates the developed oil-absorbing expansive rubber-based composite could be especially interesting for application in OBM as plugging agent.