P.J. van den Hoek

Experimental Study of Nonlinear Effects in Hydraulic Fracture Propagation(includes associated papers 29225 and 29687 )

The authors conducted fracture propagation experiments on blocks of cement paste, cement paste with sand, and a tight outcrop sandstone rock. A novel acoustic monitoring system was developed for measuring the fracture profile and radius during the tests. Results of laboratory tests on cement agreed with numerical simulations based on elastic rock deformation and linear elastic fracture mechanics. Tests at confining stresses of 20 and 23 MPa gave higher pressures than predicted.

Horizontal Wellbore Stability and Sand Production in Weakly Consolidated Sandstones

Long-term stability of horizontal wellbore completions with uncemented liners in weakly consolidated to unconsolidated sandstone formations (e.g. Gulf of Mexico, Nigeria) remains an area of concern. This paper presents the results of dedicated polyaxial cell laboratory experiments addressing this issue. In addition, the influence of rock failure in the near-wellbore region on well productivity was studied. Large blocks of a weak artificial sandstone were prepared. A hole was drilled in these blocks, and production conditions at various values of in-situ stress, drawdown and watercut, both in the absence and presence of a liner, were simulated. During testing, the hole was kept at a horizontal position in order to realistically simulate the influence of gravity forces on the movement of sand debris. The process of hole failure and restabilisation was continuously monitored by an endoscope coupled to a videocamera. The experimental results show that in the presence of a slotted liner, and in the absence of watercut, rock failure leads to a gradual annulus fill-up with loose sand, eventually resulting in a stable configuration in which only a small fraction of the far-field stresses is transferred to the liner. These results are further supported by elasto-plastic calculations. Rock failure around the liner is shown to have only a minor effect on productivity. This result implies that rock failure around uncemented liner completions will generally not be noticed at the wellhead. The introduction of a small (<5%) watercut resulted in massive sand production and subsequent liner collapse. This can be explained by the fact that watercut destroys capillary cohesion, thereby destabilising sand arches over the slots.