Efficient Stimulate Long Horizontal Well of Carbonate by Accurately Acid Placement Technology: Case Study from Deep Carbonate Reservoir, Sichuan, China

Abstract

The length of horizontal wells in deep carbonate reservoir (5000∼6000m) in Sichuan Basin of China has exceeded 2000 meters. Both generating acid-etched fractures and acidizing the dispersed pay-zone are the key to achieving optimal production. The complex conditions of multilayers make it difficult to use open hole packers, instead, sliding sleeves are used in liner completions. However, the layers show strong heterogeneity, which makes it difficult to realize efficient acid stimulation. This paper presents a technology that stimulates such wells to achieve full acidizing of drilled reservoirs, and optimize the fracturing fluid system and acid-fracturing parameters to achieve longitudinal fracture penetration based on the sweet spot recognition. Firstly, in order to describe reservoir characteristics and identify sweet spots along the well, an evaluation model was established based on the correlation evaluation parameters of the gas-bearing properties, petrophysical properties, natural fractures, and fracability. The high sweet spot coefficient (HSSC) section and low sweet spot coefficient (LSSC) section were recognized. Then, based on the section classification and the optimal fracture spacing, the position of sliding sleeves is optimized. Sleeves were mainly set at the highest points of sweet spot coefficients in the nearby section. For different sections, different acid stimulation strategies are proposed respectively. For the HSSC, it is necessary to increase the coverage of acid in the wellbore to realize the efficient acid stimulation of sweet spots alone the wellbore. However, the LSSC sections are mainly caused by the deflection of well, creating artificial fractures and making fractures propagate along the height direction into TL43-3 are needed. In HSSC sections, the acid coverage increases with the increase of acid volume and injection rate, and it is preferred to use temporary plugging to obtain the wide acid coverage when half of the acid is pumped making the two fractures obtain efficient stimulation with the acid absorption ratio of 1.47 and acid coverage of 207.1m In LSSC sections, it is still difficult to make artificial fracture break through the upper and lower layers by only use gelled acid even if the acid injection rate is greatly increased. A high viscosity fluid was introduced in acid stimulation treatments could make artificial fracture vertical propagating to break through the upper layers of 16 MPa stress difference with injection rate ≥ 5 m3/min and volume of 100m3. The productivity of 5 applicated wells increased 66.28%, which proved that the stimulation strategies of HSSC sections and LSSC sections are successful.