Sheng Mao

Key issues and investigation of horizontalwell drilling and multistage fracturing in shale gas reservoir

Unconventional shale gas is rapidly increasing as an available source of natural gas in the worldwild. Horizontal well drilling and multistage fracturing are two principle techniques of shale gas resources production. Benefiting most from these two techniques, the United States, Canada, and China have become only three countries to produce shale gas in commercial quantities. The key technical issues emerged in the horizontal drilling and fracturing involve the rock fragmentation issues, wellbore trajectory optimization and control, wellbore stability, cementing and its quality control for long horizontal lateral, and hydraulic fracturing design. Although some progress associated with those technical issues have been achieved in China, there are still many geologic and engineering challenges: (1) the low rate of penetration and fast bit wearing for both PDC and Cone bits are still the primary issues, which makes hard achieve the “one-trip” drilling; (2) wellbore collapse and lost circulation are serious with a certain collapsing period due to natural fractures system; (3) complex multi-fractures propagation behaviors still need to be understood and the fundamentals of fracturing design should be established after undstanding multi-fractures propagation behaviors.

Mechanisms of rock breaking by swirling-round SC-CO 2 jet

Supercritical carbon dioxide (SC-CO 2 ) jet is characterized by low rock breaking threshold pressure and high rock breaking rate. Swirling-round jet, taking the advantages of both swirling and round jet, enjoys high rock breaking efficiency. By integrating swirling-round jet and SC-CO 2 , it is prospective for swirling-round SC-CO 2 jet to improve rock erosion efficiency further. Rock damage mechanisms of swirling-round SC-CO 2 jet are investigated by means of rock-erosion experiments and scanning electron microscope (SEM) observation. The selected core samples include shale, dolomite as well as sandstone. For shale, no macro-deformation occurs and microscopic damage is discovered, forming tiny cracks. For dominate and sandstone, erosion holes form, and crystal particle cleavage as well as inter-particle rupture occur. According to the observation results, the rock damage mechanisms of swirling-round SC-CO 2 jet are divided into two aspects. On the one hand, swirling-round SC-CO 2 jet breaks rock by combining axial impinging, radical tension and circumferential shear. On the other hand, with characteristics of low viscosity, compressibility and strong diffusivity, numerous micro fractures are caused by SC-CO 2 , and existing fractures are expanded, which led to fractures joining together and massive rocks split off.

Shale rock fragmentation behaviors and their mechanics by high pressure waterjet impinging

High pressure waterjet technology is a potentially advanced technology to improve the drilling efficiency of shale rock formation. The success of waterjet drilling requires more insight into the rock fragmentation behaviors and their mechanics. In the present study, a series of submerged waterjet impinging trials were carried out on the outcrop shale rock collected from the Longmaxi Formation, Sichuan Basin, China. Simultaneously, the sandstone was selected as the candidate of the control group. The macro fragmentation behaviors and their micro structures were acquired by the scanning electron microscope (SEM) and computed tomography (CT) techniques. The rock mechanics and mineral composition before and after experiments were measured by the standard ISRM method and X-ray diffraction, respectively. Particularly, the grain size distributions of cuttings were obtained by a laser particle size analyzer. Results show that the efficiency of shale waterjet drilling is ultralow, which the rate of penetration and the ultimate drilling depth cannot reach the half of those sandstone. SEM imaging shows that shale is ultra-tight in which the illite clay weaves into a network and the dolomite embeds into the network. It just likes a concrete material resulting in the high compression strength. Since the static pressure of waterjet is lower than the compression strength, the common compression-shear failure does not happen, but only the tensile fracture occurs and makes the rock spallation for the initial crushing. With the penetration depth increasing, the waterjet impinging force is not able to make the rock tensile or shear failure. Only the particle erosion becomes a dominant failure mechanism for shale rock where the mineral particles are removed or crushed from rock matrix as fine cuttings. This work provides the theoretical guide for improving the shale rock drilling efficiency by water jetting.