Using an approach based on integrating rock and soil mechanics principles, a new model is presented for computing not only the onset of sanding but also the episodic nature of sanding and the quantity of sand as a function of rate of change in pressure, magnitude of pressure change, depletion and watercut. Unlike conventional approaches that simply relate sanding to shear failure in the rock mass, the proposed approach allows for the gradual disaggregation of the rock into sand grains and thereafter properly accounts for its considerable resistance in accord with the effective stress principle and consolidation theory. Within such framework, sanding is considered to occur only when seepage conditions exceed the resistance from intergranular friction, arching, and mechanical and chemical cementation. The significant added value using such an approach in oil and gas projects is shown by examining several field applications which include validation (Class A prediction).
1. INTRODUCTION
Selection of the wellbore completion type is a major design issue as it directly impacts the associated capital and operating costs and short- and long-term production potential. This issue is particularly significant in the offshore environment where the costs and risks are high. The common methods of completion for sand prone reservoirs include cased and perforated, slotted liners, openhole with wire-wrapped mesh or prepack screen, cased hole and openhole gravel packs, frac-packs and expandable/compliant screens. While completion technology has dramatically advanced over the past two decades, making the right choice still remains a difficult engineering consideration. First and foremost, one must assess the sanding potential over the projected life of the well and the proposed operating conditions. The probability and severity of sanding plays a key role in the selection of completion type but the final choice is equally impacted by economical factors.
In the offshore environment, our assumed setting henceforth, the factors affecting costs, risks and ultimate selection include: water depth, wellbore length and deviation, reservoir pressure and temperature, availability of equipment and local expertise, health, safety and environmental regulations, provisions for sand handling at the surface, erosional tolerance (particularly important in sub-sea wells), size of the reserve and production target (short- and long-term) and the projected well life. The difference in capital cost among the various options can be in excess of $10 MM/well and each system can have a differing impact on the resultant productivity index. For instance, gravel packs can become plugged with fines over time or start off with a higher skin due to the completion fluids used or poor installation procedures. Also, there is always a risk that the completion may not materialize as planned, for example, incomplete or damaged gravel pack, problems in full deployment of expandable screens and its compatibility with the reservoir conditions etc. As such, complexity of options for mitigation (workovers) is often a factor that is heavily considered in the overall selection process of the optimum completion type. Other issues include the ability to shut-off water which normally prohibits use of any openhole type of completion.
