Low-porosity, low-permeability zones are being successfully stimulated in the Appalachian Area by a technique termed Mini-Massive Frac. This technique combines several techniques commonly used to increase fracturing efficiency and improve results. These include methods to:
Control fluid loss into hairline fractures.
Control fluid loss into matrixpermeability.
Control fracture height.
Achieve a deeply penetrating, packed fracture.
Use of the technique has provided productivity increases many times greater than conventional treatments, and these are proving to be sustained increases. This paper describes the mini-massive frac and its application to low-porosity, low permeability zones in other areas.
In the 1960's, a team of researchers working under the direction of MIT geologist William Brace discovered that as rock approaches its breaking point, a myriad of tiny cracks form in certain point, a myriad of tiny cracks form in certain direction. This phenomenon is called "dilatancy" and it has been theorized that when dilatancy occurs the rock strength paradoxically increases and the rock resists fracturing temporarily. In crustal rocks, ground water seeping into the tiny cracks, however, will eventually weaken the rock and cause it to rupture.
It is believed that dilatancy also occurs during hydraulic fracturing of underground formations. (We have known for years that all subsurface rocks are triaxially loaded; that is, they have a force exerted upon them by the overlaying formations. This vertical force or stress imposed on the rock causes a horizontal stress to exist within the rock system. If the rock were not confined within the earth, this vertical force would express itself by causing the rock to deform or expand its lateral dimensions. When pressure is applied from within such a system, the rock ruptures or fractures in a plane perpendicular to the least amount of stress on the system. These stresses will control the direction of the fracture and determine whether the fracture plane will be horizontal, vertical or inclined.) During a fracturing treatment pressure is applied to a rock formation. As the rock approaches its breaking point due to the applied pressure, a myriad of tiny cracks form. Dilatancy pressure, a myriad of tiny cracks form. Dilatancy then continues to occur along the leading edge of the fracture as it progresses outward into the formation. This theory of dilatancy during hydraulic fracturing can be used to explain several phenomena observed from past fracturing treatments. phenomena observed from past fracturing treatments. Some of these are:
Premature screenout in very low permeability formations.
Failure of the fracture to penetrate as far as calculated.
Failure of the well productivity to achieve predicted folds of increase.
The explanation simply is that a portion of the frac fluid leaks off into the myriad of tiny cracks and the natural fractures encountered. The fluid that leaks off is not available for fracture extension.
This leak off of fluids can be controlled with high viscosity fluids, temporary fluid-loss additives, etc. But when the frac job is complete, the cracks and natural fractures close. Another way to control this leak off is with the use of 100 mesh sand as a spearhead. The diameter of a grain of 100 mesh is .0059 inch while a grain of 20–40 frac sand has a diameter of 0.033 inch.