This paper describes characterization and its successful applications of a deep-penetration acid-fracturing approach to enhance oil recovery in low-permeability complex lithology reservoirs. A kind of special but important low-permeability reservoirs are distributing widely in west of China, in which neither carbonate nor detrial rock is main content.It is named complex lithology, which has 1/3 of quartz and feldspar, 1/3 of carbonate, and 1/3 of clay mineral. Early attempts in stimulations got miserable results.Propped fracturing failed with little bauxite placed before job termination due to premature screen-out. In addition, less HCl consumption and rock dissolving power prevents deep penetration of acid into formation. In matrix treatments this results in compact dissolution with only marginal skin decrease.
To optimize acid system for this complicated formation condition, acid rock reaction experiments of gelled acid and emulsified acid were accomplished to get full understanding the kinetics of acid reaction with complex lithology cores. Acid conductivity experiments were conducted with a unique conductivity instrument using samples designed to simulate deep-penetration acid fracturing process. The lab results indicate acid reaction mechanism is quite different both from carbonate and sandstone that is not controlled by surface reactivity or mass transport, but by both of these two reactions modes in balance. A good way to improve conductivity is alternating stages of acid fracs and closed acidizing with a multi-component acid, which is available to dissolve quartz, feldspar and clay.
More than 63 treatments with new technologies in Yumen, Qinghai oil field have been performed with encouraging results, 74.5% efficiency with accumulated incremental oil of 254,900 tons. For example, well L8 had original oil production of 546 BOPD, and got production as much as 2016 BOPD after treatment.
Low-permeability complex lithology reservoirs account for a considerable proportion in western reservoirs in China, which is one of very important low-permeability reservoir types, such as Qingxi reservoir in Yumen, Gasi reservoir in Qinghai, Santanghu reservoir in Tuha, etc. Mineral composition in Qingxi reservoir in Yumen mainly includes1/3 of terrigenous clasolite (quartz and feldspar); 1/3 of carbonate (calcium carbonate and calcium magnesium carbonate, etc.), and 1/3 of clay mineral; mineral composition in Gasi reservoir in Qinghai mainly includes 30–39% terrigenous clasolite (quartz and feldspar), 28–36.7% carbonate, and 24.3–30% clay mineral. Obviously, if it is distinguished as carbonate, sandstone or mudstone, it is not very accurate and not helpful to stimulation selection [1, 2]. Therefore, this special reservoir is named as complex lithology reservoir, that is, the proportions of quartz and feldspar, carbonate and clay mineral in reservoir mineral composition are very close or relatively equal.
In the past, all stimulation methods had been tried in low-permeability complex lithology fracture-type reservoir, such as hydraulic fracturing, acid fracturing and matrix acidizing. If hydraulic fracturing is applied, due to existing of natural fractures and high plasticity of rock, sand premature screen-out always occurs with high failure rate. If acid fracturing favorable in carbonate reservoir is applied, especially with HCl acid system, acid etched fractures have low conductivity and stimulation results are not good. If sandstone matrix acidizing is applied, the effectiveness ratio is low and stimulation effective period is short. Studying stimulation technology suitable for complex lithology, finding proper technical methods and optimizing corresponding liquid systems are an important technical path to further improve the stimulation results in complex lithology reservoirs.