Investigation of Gas Flow Hindrance due to Fracturing Fluid Leak Off in Low Permeability Sandstones

Open Access
Author:
Odumabo, Sijuola M
Graduate Program:
Energy and Mineral Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
March 26, 2013
Committee Members:
  • Zuleima T Karpyn, Thesis Advisor
  • Luis F Ayala H, Thesis Advisor
  • Yilin Wang, Thesis Advisor
Keywords:
  • hydraulic fracturing
  • low permeability formation
  • saturation
  • leak off
Abstract:
Hydraulic fracturing has become a necessary practice in order to attain economical gas flow rates from low permeability formations. During and immediately following the creation of a fracture, high injection pressures cause fracturing fluid to leak off into the adjacent matrix. Such an occurrence can have ramifications which counter the function of a fracture. This work focuses on the effect of leak off in low permeability sandstones while quantifying subsequent impact on gas flow. In particular, the effects of leakoff volume and the length of the shut-in period following fracturing activities are investigated. Gas flow hindrance is then explored in terms of permeability changes. Sandstone outcrops were obtained and cored to 1.5-inch diameter and 7-inch length cylindrical samples. Leak off was simulated by injecting distilled water into one face of a sample designated as the ‘fracture face.’ Leakoff volumes were categorized into high and low leakoff while shut-in times ranged between 24 hours and 312 hours. A series of base and regained permeability tests were conducted prior to and after the simulation of a leak off to monitor any improvements. X-ray computed tomography (CT) was used to visualize the speed and extent of the imbibing front at various time intervals. Saturation profiles were developed from the resulting images. Gas flow hindrance caused by the leak off of water-based fracturing fluid is mitigated by shut-in time in that it favors spontaneous redistribution of the fluid deeper into the formation. Experimentally, it was observed that an increase in leakoff volume reduces effective permeability to gas while an increase in shut-in time increases effective permeability to gas. Regained permeability results suggest that the dominant of these competing forces is the leakoff volume. A greater leak off is more damaging than a reduced shut-in time. This imbalance highlights the key determining factor behind gas flow hindrance due to fracturing fluid leak off – fluid saturation in the neighborhood of the fracture. The saturation in question is a direct result of the fluid left behind following spontaneous imbibition of fracturing fluid during the well's shut-in period, and subsequent flowback procedures. Saturation in this context is a function of shut-in time and leakoff volume. A decrease in saturation within the invaded zone is generally favorable as it increases effective permeability to gas. Furthermore, lower formation permeability slows improvements to gas flow due to lower mobility of the invading fluid despite expected higher capillarity. It was found that the properties of the formation play a significant role in determining regained permeability. An increased shut-in period is typically advantageous but it does not always favor regained permeability in formations with depressed relative permeability curves. In these types of formations, shut-in time also allows spreading of the invading fluid which improves effective permeability to gas in the invaded region. However, the improvement in permeability is attenuated due to the depressed nature of the formation’s relative permeability curve. As the invaded zone expands with time, the improvement in permeability within it becomes inconsequential when compared to the extent of invasion. Hence, there is a point after which further imbibition becomes detrimental. Leakoff volume and shut-in time are variables that work differently to dictate saturation distribution in the neighborhood of the fracture. Saturation within the invaded zone and characteristics of the formation’s relative permeability curve may be the key determinants of gas flow hindrance following hydraulic fracturing activities. Conditions of saturation distribution and petrophysical properties vary between formations. This may explain the lack of correlation between volume of flowback water and gas production in the field.