Excessive fluid loss during hydraulic fracturing of high-permeability formations can produce several problems that can affect the execution of the job, cleanup and post-production of the well. Presented are new fluid-loss additives based on optimized blends of novel particulates and starch to control spurt loss in highly permeable formations under high shear rates. Laboratory data are presented where the effectiveness of the new additives is compared to existing fluid-loss additives in different fracturing fluids under dynamic conditions. Laboratory results also show the effect of these new fluid-loss additives in making spurt-loss less sensitive to the rock permeability, which in highly permeable rocks yields a more predictable leakoff. This facilitates predictable design and execution of the jobs. Laboratory data also show that the matrix damage caused by the invasion of borate-crosslinked fracturing fluids is significantly lessened when the new fluid-loss additives are used. Finally, a field case study is provided, illustrating the use of these new additives in combination with low-concentration borate-crosslinked guar fluids. The result was predictable response during execution, reduced cleanup times, and significantly improved production.
Fluid-loss control in fracturing of highly permeable formations is important for several reasons. The most obvious is that excessive fluid volume lost to the formation increases the fluid costs. In addition depending on the type of fluid, the fluid lost can damage the matrix around the fracture - potentially lowering the initial production rates. Also, the mere action of pumping larger volumes of fluid to create a fracture volume generally results in increased well cleanup times. All these problems can be minimized by effectively controlling spurt loss.
There are other less obvious problems associated with fracturing high-permeability formations. For example, to successfully execute fracture-tip screenout (FTSO) designs it is necessary to accurately determine the pad volume. Insufficient pad will lead to a premature screenout and a reduction in fracture penetration, while too much pad will prevent the tip screenout from occurring, eliminating the conductivity benefits of the FTSO design. Pad volumes can be determined accurately only if the leakoff properties of the formation are thoroughly known. However, these depend heavily on the permeability of the rock, which varies spatially away from the wellbore. Normally only an average permeability for a well is known and variations in permeability are unknown. Effective spurt-loss control helps minimize the impact of permeability variations on leakoff rates by reducing the effective permeability of the rock to predictable low values.
Spurt loss, or the amount of fluid lost when a rock segment first becomes exposed to fracturing fluid under pressure, occurs close to the tip of the advancing fracture. The width of the fracture at the tip is the narrowest and, therefore, the shear rates are the highest. High shear rates tend to limit or even prevent the formation of external filter cakes; therefore, it is not possible to rely on external filter-cake deposition to control spurt loss under these conditions. However, it is necessary to develop new fluid-loss technology that can perform efficiently under the high shear environment of the fracture tip. Novel particulates have been identified that are able to reach the fracture surface quickly under high shear rates, bridging the pore throats at the fracture surface. Subsequently, starch or other particulates can deposit and form a low-permeability filter cake.
In this paper it is shown how optimized mixtures of novel particulates and starch can effectively control spurt loss under high shear rates. Laboratory data and a field case history are presented to support this conclusion.