Horizontal Well Sectional Frac Technique with Dual-Retrievable Packers in Daqing Oilfield

Abstract

Horizontal well technology is a technological revolution in oil industry, and it is one of the most important methods of thick layer sustainable production and marginal low permeable formation exploitation in Daqing Oilfield. It is commonly believed that fracturing is an effective way to recover oil from thin, low porosity and low permeability sands. However, the existing frac technique in the horizontal wells - the limited entry frac is a kind of commingled frac method that cannot guarantee the fractures' opening and extension uniformly owing to heterogeneity formation. A novel sectional frac technique with dual-retrievable packers by separately stimulating the specific intervals one by one has been developed to recover low permeable reservoirs in the horizontal wells. Sectional frac string chosen includes high pressure, high temperature tools which have been used in several wells application as well as long term tested in a laboratory environment to differential pressures of 7300 psi at 190 °F with functions of stuck-preventing, stuck-releasing and reverse sand flushing. Five intervals treatment can be done in one trip. Up to now, this technique has been applied on 55 horizontal wells in Daqing Oilfield and 281 intervals were fractured in the field. It usually takes one to three days to frac a horizontal well according to the numbers of target reservoirs. The initial oil production after using the sectional frac technique in horizontal wells is 5.7 times higher than that in vertical wells, and 1.04 times higher than that by using the limited entry frac technique in horizontal wells. The current oil production after using the sectional frac technique in horizontal wells is 4.3 times higher than that in vertical wells, and 1.72 times higher than that by using the limited entry frac technique in horizontal wells. Field runs and lab testing have confirmed that sectional frac technique is a viable option in recovering low permeable reservoirs in the horizontal wells. Introduction Daqing oil field is a giant, heterogeneous, multilayer sandstone oilfield. After long-terms of water flooding, 95% of main pay zones have been in production and water cut has exceeded 91%. The development of the oilfield has also aimed at relatively thinner and less permeable layers to maintain stable production. For water-flooding oilfields, the thin and poor reservoirs' development strategy is facing a challenge after oilfields has gone into high water-cut period. Usually, many thinner pay-zones are laminated within the thicker reservoir interval penetrated by the well. Thus, it becomes difficult to optimize the injection strategy for individual zones, even though their flow properties may be different, requiring different injection pressures, different additives, or different polymer concentrations. It is commonly believed that horizontal well has larger contact area in the thinner reservoirs compared with the vertical wells. Moreover, fracturing is an effective method to increase conductivity near the wellbore and improve the horizontal wells' performance. Due to the existed deflecting interval and the horizontal interval with target formations, operation with running frac pipe string and string's force status are far different compared with those in vertical wells. Accordingly, conventional frac technique - the limited entry frac is not suitable for stimulating horizontal wells in the Daqing Oilfield. Large size coiled tubing and coiled tubing with through-tubing packer are less used for horizontal wells in Daqing, which are popular in other places. The average horizontal interval length of horizontal wells is 583.7m in Daqing. There are some difficulties met in the horizontal well frac that are listed as follows: High risk of operation: The actual track of horizontal wellbore is so complicate that it is difficult to run the string smoothly in the well in case of string-stuck accident happened.