Effects Of Fracture Conductivity On Effective Fracture Length In Fractured Vertical Well In Tight Gas Reservoir

To develop low permeability gas reservoirs, hydraulic fracturing is normally required. By pumping a large volume of fracturing fluid mixed with propping agents or proppants, creates a long and highly conductive fracture, resulting to improved permeable path to well bore thereby improving single well production and ultimate well recovery. In field production, researchers have found productivity of low-permeability wells are far less than predicted after fracturing treatment, and the major reason is that fracture lengths achieved are less than designed lengths. These short apparent fracture lengths can result from excessive height growth, tip screen out, poor proppant transport, proppant flowback, or a combination of these factors. It is known that fracturing fluid that remains in the fracture and formation after a hydraulic fracture treatment can decrease the productivity of a gas well, reducing the relative permeability to gas in the invasion zone. Therefore, in order to increase gas rate, the remains of strata fracturing fluid must be ensured as little as possible. However, the relationships between conductivity and effective fracture length are not fully understood. Therefore, study on the relationship between conductivity and its effective length, giving reasonable flow capacity and effective length and later optimized fracturing design are significant to achieving efficient development of tight gas reservoir.In the design of hydraulic fracturing, we ordinarily propped fracture length(design length) is defined as the effective length. However, tight gas reservoirs generally have ultra-low water saturation phenomena, easy to form a water phase trapping regions impeding the flow of gas to the fracture after hydraulic fracturing. Therefore, the effective fracture length is less than the designed length. In this paper, the effective fracture length is defined as the fracture practical contributions to the production(Gas flow into the crack length).Based on static and dynamic geo-characteristics of Shaximiao Formation in Zhongjiang, analysis results showed that the main reason for low-yield wells in the area are low gas relative permeability in invaded zone, and the effective length is less than the designed length. Sand fracturing in Well S1, and pressure build-up test technique applied a week later, results indicated that extrapolation formation pressure was 47.6252 MPa, and pressure coefficient was 1.68, close to initial formation pressure. However, the analysis formation effective permeability is only 0.00468×10-3μm2, far less than in-situ value, which is 0.18×10-3μm2, and effective fracturing length is 17 m, just 8.9% of designed fracturing length, which verified the existence of water trapping in tight gas reservoirs. Invasion of fracturing fluid increase the water saturation in invaded zone, reducing the relative permeability of gases, contributing to short effective length. The short fracturing length is the reason for actual capacity being lower than expected production capacity. However, tight gas reservoirs is ultra-low water saturation conventionally, actual water saturation is less than the minimum water saturation. As a result, we are just able to increase fracturing fluid flowback rate to extend effective fracturing length and enhance gas productivity. Using reservoir simulation technique effects of varies fracture conductivity in different formation permeability, half length, different filtration of different cracks and different cracks into injury rates conditions on the effective length are studied. Cumulative distribution curve by gas flow into the cracks under various conditions of flow capacity and different effective length of production time were compared to analyze the distribution of gas along the fracture and influence of conductivity on effective length under a variety of factors.Studies have shown that Shaximiao Formation has good geological resources and prospects for exploration and development of. Invasion of fracturing fluid caused significant damage to gas relative permeability in invaded zone., effective fracture length is less than designed length in the early stage. As production time increasing, the effective length can extend to nearly designed length after 1-year production. Although gas flows along the entire crack, but residual fracturing fluids in invaded zone cannot flow back entirely. Distribution of water saturation affects the gas velocity to the crack, determining the length of effective crack. Minor changes of gas flow efficiency will affect effective fracture length significantly. In other words, greater fracture conductivity, more thorough fracturing fluids flowback, longer effective length and larger production will be gotten as a result.