A Novel Clay Control and Conductivity Approach in Water Sensitive Tight Oil Reservoirs in China During Proppant Fracturing
Authors: Wang Zichao , Zhang Wendi , Wu Changhong , Song Shunyao , Liu Xuewei 
Published: March 2023

Unconventional oil formations require high fracture contact with reservoir to enhance productivity. These target reservoirs have significantly tight characteristics with low permeability and nano porosity, that show adverse water sensitivity and proppant embedment. Clay swelling and dispersion due to water sensitive clay minerals in the producing formation can substantially reduce rock permeability and proppant embedment in high clay minerals reduce retained conductivity resulting in reduced well productivity. The effects of water based fracturing fluids in water sensitive formations have been extensively investigated which led to development of different types of clay stabilizers based on type of clays, bottomhole temperature, and clay protection time. A novel clay stabilization liquid additive has been successfully introduced for permanent frac face clay protection, while an SMA (surface modified agent) additive was used for proppant pack protection.

The frac strategy is to enhance propped fracture productivity and fracture conductivity while protecting the fracture face, instead of using high slickwater volumes to create shear fractures. Crosslinked fluid was utilized to control the fluid leak-off into formation and increase proppant delivery capacity. The new permanent clay control additive is a cationic polymer with ultra-low molecular weight and is compatible with borate and zirconium crosslinkers. A new concentrated clay control additive prepad was used during the treatment which reduced pad leakoff damage. The conductivity enhancer SMA with nano coating was used on the proppant pack to mitigate embedment damage and increase the proppant pack conductivity.

Designing and optimizing a particular type of clay stabilizer requires identifying the types of clay minerals present in the formation and their relative concentrations. For the formation in this study, clay fraction was 9∼11% with over 75% illite/smectite mixing layer. Core flood testing provides the most authoritative data upon which to base recommendations and results showed regained perm at +75%. Accordingly, a special clay stabilizer concentration was chosen to be used in the fracturing fluid. Based on reservoir closure stress and the conductivity test results, an SMA treatment concentration was chosen. Then fracture density and cluster spacing was optimized using geological data in a 3D frac model with log interpretation profiles. The productivity of fracturing treatments performed using this innovative approach resulted in negative skin factors and production ratios that exceeded expectations in water-sensitive and high clay content reservoirs.

This paper provides a novel approach and frac strategy on the water sensitive and soft rock formations. Lab testing and field implementation of this novel approach on proppant fracture treatments are detailed in this paper. Post treatment pressure matching was conducted to predict the final fracture geometry and then analysis was performed using the created fracture geometry and actual flowback data to further validate the incremental production increase after the treatment.