Horizontal-Wellbore Stability and Sand Production in Weakly Consolidated Sandstones

Abstract

Summary Long-term stability of horizontal wellbore completions with uncemented liners in weakly consolidated to unconsolidated sandstone formations (e.g., the Gulf of Mexico and Nigeria) remains an area of concern. In this paper we present the results of dedicated polyaxial cell laboratory experiments that address 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 water cut, 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 re-stabilization was continuously monitored by an endoscope coupled to a video camera. The experimental results show that in the presence of a slotted liner, and in the absence of a water cut, rock failure leads to gradual annulus fillup 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%) water cut resulted in massive sand production and subsequent liner collapse. This can be explained by the fact that a water cut destroys capillary cohesion, thereby destabilizing sand arches over the slots.