Application of Sand-Ramp Fracturing Pumping Mode in Unconventional Shales Stimulation

Abstract

Sand-slug fracturing has been the main fracturing pumping mode due to the tightness of shale. This mode makes it easier to inject proppants. However, it may cause poor connectivity in the middle brittle formation due to the discontinuous propping. This paper describes an attempt to fracture the unconventional shales with conventional sand-ramp fracturing pumping mode. The results show that good effect is achieved compared with the sand-slug fracturing mode used in the adjacent wells. Shale reservoir reconstruction has large construction displacement and high pressure, and it adopts fracturing technology of slickwater and linear glue1. In other blocks, Sand-slug mode causes large amounts of slickwater injecting into the formation. The role of water, in addition to carrying sand and making seams, will also cause "water lock" in the micro-fissure, reducing the gas permeability. Therefore, the large amount of liquid is not good to the reservoir. The Sand-ramp modes using less fluid and higher sand content2-3, resulting in a greater height and length of the fractures. By using a small amount of proppant, Sand-ramp mode can also achieve high conductivity4, communicating the natural cracks5-7 at the same time. Two wells were designed for Sand-ramp mode in the test area. Based on the understandings on geological characteristics and formation property, the sand-ramp fracturing pumping mode was designed. Two of six wells in the pad were selected to apply this mode. To maximize the stimulated reservoir volume, slickwater accounted for 40% to 60% of the total injected fluids. 100-mesh quartz sands were pumped in priority to improve the complexity of fracture. Then, the 40-70 mesh ceramsites was pumped with crosslinked gel to support the primary, secondary and natural fractures. The pumping rate is 12-13 cubic meters per minute and no acid is used throughout the whole pumping process. The maximum proppant concentration of sand-ramp reached to 480 kilogram per cubic meters, which was much higher than that of sand-slugs. As a result, good propped fractures were obtained. Since no fluid sweep was used after the sand-slug, the average fluid injection per stage is declined by 27%, but the average sand injection volume was increased by 17%, which significantly reduced the cost and the possible influence to environment. With the sand-ramp mode, the highest test production of the block was up to 278500 cubic meters per day. This well produced 35 million cubic meters of shale gas in 270 days. The practicability of the sand-ramp pumping mode used in unconventional shales is proven to be positive, especially in the formation with high horizontal stress difference. However, the development result needs to be continuously studied.