The analgesic effects of tDCS are dependent on spinal cord excitability. This work provides insight into top-down modulation during acute pain and temporal summation. This knowledge may explain why tDCS has a higher analgesic efficacy in chronic pain patients.
Gas breakthrough because of coning is very common in Bahrain field particularly in naturally flowing oil wells. It escalates gas flow and reduces oil rate as cone reaches the perforations. Consequently, wells gas out and recovery of oil below the GOC becomes very difficult. Flow stopcocking with time cycle control has been introduced to restrain gas cone breakthrough as conventional surface bean reduction approach proved ineffective. Case studies show that stopcocking enables a better control on draw down, prevents cone breakthrough and improves oil recovery. In addition, wellhead pressure data reveals that smaller surface bean causes increased liquid fall back and reduces stopcock flow efficiency. The case studies brought to light a very important fact of liquid reentry into formation and consequent slug height loss. Study of borehole flow dynamics indicates liquid diversion back and forth annulus and tubing during shut in cycle. A mechanistic model has been developed to help understand the flow behavior and optimize the system operation. Simulation shows that the performance of downhole flow control plays very important role in liquid accumulation inside tubing and annulus. Moreover, completion geometry has significant role in gas accumulation, fluid segregation and slug height build up. To optimize the stopcocking flow, a completion design that alleviates the drawbacks of the typical configuration has been discussed. Introduction Gas coning is prevalent in naturally flowing Mauddud (B) zone wells in Bahrain Awali field. Mauddud is 100 to 120 ft thick limestone reservoir of Middle Cretaceous age. Although originally undersaturated, exploitation for over seven decades and ongoing gas injection for over three decades for pressure maintenance have caused a large gas cap in this reservoir (Fig. 1). The reservoir is highly faulted and the adjoining layers Ac, Ca and Cb (Fig. 2) are believed to be in communication with Mauddud in many wells. Wells completed in Ac, B, Ca, Cb zones within the gas cap limit, usually suffer from gas coning. These wells gas out in several weeks to few months of continuous production even with surface bean restrictions and are to be shut in to recede gas cone. Nevertheless, cone reappears within a few weeks of flow period post few months shut in. Flow stop cocking was first started in 1994 to economically improve recovery of underlain crude below Mauddud gas cap. The system operates with a flow line on-off control valve (Fig. 3), which is triggered by a time cycle controller. Short flow and shut in time cycles disallow flow stabilization and hinder gas cone breakthrough. Case studies illustrating the system flow behavior and the optimization approach undertaken based on the surface pressure recordings have been discussed. A mechanistic model on stopcock flow has been built on similar principles as the dynamic slug flow modeling in plunger and intermittent gas lift systems in Refs. 1 through 4. Ref. 4 develops a concept of threshold GLR based on the completion geometry. It also raises concern on the plunger lift efficiency in case of liquid column formation in annulus during shut in period. This paper addresses the effect of downhole bean and distribution of liquid slug in tubing and annulus. A completion configuration to improve the stopcock performance has also been presented.
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