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 farfield 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.
Reliable predictions of sand production potential are required to make realistic sand production management and contingency planning possible. Unnecessary application of sand exclusion measures results in increased completion costs and considerable loss of well productivities. Further, sand prediction may assist in selecting the most attractive sand control techniques.
Over the years, a large number of models for sand production prediction have been developed, see e.g. Refs. 2-9. These models generally focussed on the prediction of the onset of sand production. A new conceptual model for initial sand production prediction is presented in an accompanying paper. However, in many situations a certain degree of sand failure around the wellbore, and resulting sand production, is acceptable within limits. This paper presents laboratory experiments focusing on re-stabilisation after initial sand failure around horizontal wellbores with or without uncemented liners, in weakly consolidated to unconsolidated sandstones. Long-term stability and productivity of these completion types remains an area of concern in many fields, e.g. Gulf of Mexico, Nigeria, North Sea, etc.
Large blocks of a weakly consolidated (0.46 MPa cohesion) artificial sandstone were prepared, and a hole was drilled in these blocks. The effects of in-situ stress, flow rate (drawdown), watercut, and completion type (open hole, predrilled liner, slotted liner) on stability and productivity were investigated. 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-stabilisation was continuously monitored by an endoscope coupled to a videocamera. The liners used in the tests were made out of (transparent) plexiglass in order to monitor the failure process around the liner with the endoscope.
The experimental results show that in the absence of a liner, increasing the far-field effective stress leads to gradual hole closure instead of a sharply defined 'failure'. In the presence of a slotted liner, and in the absence of watercut the endoscope images showed a gradual annulus fill-up by loose sand with increasing far-field stress.
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