Li, Feng (Southwest Petroleum University) | Xie, Xiong (CNOOC-Shenzhen) | Huang, Li (CNOOC-Shenzhen) | Zhou, Luyao (CNOOC-Shenzhen) | Chang, Botao (Schlumberger) | Wang, Chao (Schlumberger) | Wang, Fei (Schlumberger) | He, Chengwen (Schlumberger)
In China, the main sandstone reservoir M of the LF oilfield entered the mature development stage with high water cut (average 93%) and 66.1% recovery. Remaining oil exists vertically in the H layer at the top section of this massive bottomwater reservoir and laterally at margins of current development area with less well control. The H layer consists of several thin (0.5 to 2 m) sand sublayers interbedded with calcareous tight sublayers with low permeability; the effective oil drainage radius of single borehole is 100 to 150 m. Maximum reservoir contact (MRC) technology was employed to increase drainage area and volumetric sweep efficiency for optimal production and recovery to rejuvenate this mature reservoir.
In an original hole with 98 to 99.9% water cut targeted for a workover operation, two new laterals were sidetracked to comprise a three-lateral MRC configuration with openhole completion to develop the SL1 target sublayer of the H layer. The success of MRC wells depends on an efficient openhole sidetrack and azimuth turning. Moreover, multilaterals need to precisely chase the sweet zone in the reservoir. Drilling into overlying shale causes borehole collapse, and penetrating the underlying tight zone causes fast bottom water breakthrough. Low resistivity contrast increases the difficulty of distinguishing the target zone from the shoulders. Sparse well control and limited seismic resolution bring high structural and stratigraphic uncertainties. Accordingly, effective services were equipped to overcome these challenges to achieve the required engineering and reservoir objectives. The new-generation hybrid rotary steerable system (RSS) tool provides stable, rapid, and accurate steering control, even with high dogleg severity, to achieve engineering objectives. With a balance between resolution and depth of investigation (DOI), high-definition deep-looking resistivity inversion uses the Metropolis coupled Markov chain Monte Carlo method to clearly identify multiple layers (more than three) within an approximately 6 m DOI, formation resistivity distribution, anisotropy, and dip, even in this low-resistivity-contrast environment. Reservoir details could be clearly unveiled to help MRC lateral steering along the thin target. Furthermore, a wide-range-displacement electrical submersible pump (ESP) helps optimize openhole performance.
Six new laterals were drilled in three MRC wells. Hybrid RSS tools provided 100% openhole sidetrack success rate, and laterals were turned laterally with 15 to 70° azimuth change and 200- to 570-m displacement to maximize the drainage area. Deep-looking inversion revealed high-definition reservoir details by delineating three key boundaries and four adjacent layers' profiles simultaneously and identifying target zone's thickness and property variation. The target sand is 0.5 to 2 m thick with resistivity of 2 to 9 ohm-m, surrounded by interbeds with resistivity 8 to 10 ohm-m. Within the refined 3D reservoir model, the horizontal laterals efficiently chased the top section of effective target sand while avoiding high-risk shoulders. Total 4298-m horizontal footage was achieved in six laterals with net-to-gross 91% in the SL1 thin, low-permeability reservoir. With the proper ESP configuration, approximately 688,500 bbl of oil have been produced as of December 2018. Especially in two workover MRC wells, after approximately 2.5 years of production, the current water cut is 96 to 97%, lower than water cut (98 to 99.9%) before the workover operation, and daily oil production increased significantly.
Integrated drilling, logging, and production services provided MRC efficiency to rejuvenate this thin, low-permeability and low-resistivity mature reservoir.