Bedding interfaces occur in laminated rock formations at the boundary between different rock types. These interfaces can contribute to fluid leakoff during hydraulic fracturing and thus affect fracture geometry, propped surface area, and the overall hydrocarbon productivity of wells, yet they are only recently being studied. The objectives of this investigation are to empirically measure leakoff into fabricated bedding interfaces and investigate the change in leakoff introduced by fracturing fluid additives, and consequently investigate potential increases in propped surface area and productivity by using fluid additives.
A laboratory scale flow cell was developed that accurately simulates hydraulic fractures by allowing for: (i) interfaces that are independent from each other and have adjustable thicknesses that can be changed depending on the proppant being used, and (ii) fracture size and system specifications that produce the same fluid velocities as during field hydraulic fracturing, and (iii) the fluid leaked through the interfaces is continuously collected and weighted, which allows the total leakoff rate to be measured. In addition, the flow cell allows direct visualization of hydraulic stimulations as the entire system is made of visually transparent acrylic blocks.
Flow experiments, with clean fluids of different viscosities, were conducted and verified that the measurements agree with theoretical results from lubrication theory. Results of experiments with proppants verified that proppant enters the interfaces for the case when proppant diameter is several times smaller than the interface thickness. And, experiments with cellulose fibers demonstrated that the fibers bridged and accumulated in the regions near interfaces while the main fluid flow inside the fracture developed a tortuous path around these fibers, reducing the leakoff by changing the flow pattern inside the fracture.
Understanding the leakoff potential of bed parallel interfaces will allow better evaluation of fracture geometry, fluid flow and proppant transport in the fracture, the resulting propped surface area, and the potential for frack hits during hydraulic fracturing.