Thursday, December 14, 2017

Sand Control

Sand production erodes hardware, blocks tubulars, creates downhole cavities, and must be separated and disposed of on surface. Completion methods that allow sandprone reservoirs to be exploited often severely reduce production efficiency. The challenge is to complete wells to keep formation sand in place without unduly restricting productivity. 

 Unconsolidated sandstone reservoirs with permeability of 0.5 to 8 darcies are most susceptible to sand production, which may start during first flow or later when reservoir pressure has fallen or water breaks through. Sand production strikes with varying degrees of severity, not all of which require action. The rate of sand production may decline with time at constant production conditions and is frequently associated with cleanup after stimulation. 

 Sometimes, even continuous sand production is tolerated. But this option may lead to a well becoming seriously damaged, production being killed or surface equipment being disabled. 

 Factors controlling the onset of mechanical rock failure include inherent rock strength, naturally existing earth stresses and additional stress caused by drilling or production. In totally unconsolidated formations, sand production may be triggered during the first flow of formation fluid due to drag from the fluid or gas turbulunce. This detached sand grains and carries them into the perforations. The effect grows with higher fluid viscosity and flow rate, and with high pressure differentials during drawdown. 

In better cemented rocks, sanding may be sparked by incidents in the well's productive life, for example, fluctuations in production rate, onset of water production, changes in gas/liquid ratio, reduced reservoir pressure or subsidence.

Fluctuations in the production rate affect perforation cavity stability and in some cases hamper the creation and maintenance of sand arches. An arch is a hemispherical cap of interlocking sand grains. 

Other causes of sanding include water influx, which commonly causes sand production by reducing capillary pressure between sand grains. After water breakthrough , sand particles are dislodged by flow friction. Additionaly, perforating may reduce permeability around the surface of a perforation cavity and weaken the formation. Weakened zones may then become susceptible to failure at sudden changes in flow rate.  

Predicting Sanding Potential

The completion engineer needs to know the conditions under which a well will produce sand. This is not always a straightforward task. At its simplest, sand prediction involves observing the performance of nearby offset wells.  

In exploratory wells, a sand flow test is often used to assess the formation stability. A sand flow test involves sand production being detected and measured on surface during a drillstem test (DST). Quantitative information may be acquired by gradually increasing flow rate until sand is produced, the anticipated flow capacity of the completion is reached or the maximum drawdown is achieved. A correlation may then be established between sand production, well data, and field and operational parameters.

Accurately predicting sand production potential requires detailed knowledge of the formation's mechanical strength, the in-situ earth stresses and the way the rock will fail.  Laboratory measurements on recovered cores may be used to gather rock strength data. Field techniques like microfracturing allow measurement of some far-field earth stresses. 

The earth in -situ stresses are due to many factors including the weight of the overburden, tectonic forces and pore pressure. While the vertical stresses may be estimated using bulk density logs, horizontal stresses are more problematic. Accurate estimates of horizontal stresses are integrated with logs and , using a geologic model, a continuous profile of earth stresses is created. Various geologic models have been developed to cope with the different environments encountered. Reservoir pore pressure information is also needed an this may be estimated using wireline formation testing tools or DSTs.

 Once it has been established that at planned production rates sand is likely to be produced, the next step is to choose a completion strategy to limit sanding. A first option is to treat the well with "tender loving care" , minimizing shocks to the reservoir by changing drawdown and production rate slowly and in small increments. Production rate may be reduced to ensure that drawdown is below the point at which the formation grains become detached.

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