A laboratory experiment has been conducted to study the behavior of water coning under a horizontal wellbore having a stinger inserted. A large Hele-Shaw cell was used to perform the experiment. The length of the stinger employed, oil viscosity, and initial thickness of oil column were varied to investigate their effects on the optimal length of the stinger.
Results obtained show that systems without stinger yield better early recovery performance. Installing a stinger flattens the cone of water and reduces the rate of increase in water cut, leading to an improved recovery significantly. It is found here that the optimal stinger length is about 0.3 of the length of the horizontal wellbore section, regardless of the initial thickness of oil column and water/oil mobility ratio.
Producing oil from reservoirs underlain by bottom water almost always faces water coning problems. Water coning itself is referred to as the process of the formation of a water cone within the oil zone beneath the wellbore. The water cone represents the region of the oil zone that has been invaded by bottom water. This cone is formed due to an appreciable pressure gradients that exist between the plane of oil/water interface (i.e. WOC) and the wellbore. In an isotropic reservoir penetrated by a vertical well, the greatest pressure gradient exists along the shortest distance between the lowest entry point at the wellbore and the WOC. The gradients, as well as the velocity of water particles at the bottom of the oil zone, decrease as the radial distance from the wellbore increases. So, the upward movement of the oil/water interface results in water coning formation.
At higher pressure gradients or higher rate, the water cone becomes steeper and eventually breaks at the wellbore. The period of time required by bottom water to reach the wellbore at the first time is called as the breakthrough time. Afterward, the water and oil flow into the well simultaneously. Once the water appears at the surface, the production is followed by a rapid increase in water cut and extra money has to be spent for oil/water separation processes and water disposal.
The detrimental effects of severe water coning on an oil reservoir are that the water cone tends to block oil flow paths in a region around the wellbore, oil production decreases sharply with time, and thus producing life of the well is shorter, leading to an inefficient recovery.
The use of horizontal wells in bottom water drive oil reservoirs has been demonstrated in offering some advantages, such as longer breakthrough time and a higher productivity, over conventional vertical wells. However, existing pressure gradients within a horizontal wellbore create varied pressure drawdown and non-uniform flux as well. A numerical study has shown that water breakthrough occurred at the heel end of the well. The same phenomenon has also been demonstrated by a laboratory investigation, which further exhibited that a considerable length of the upstream section of the wellbore never contacted by bottom water at even a very high water cut.
Various improved horizontal well completions for restoring the well productivity have recently introduced. One of the method invented is to redistribute pressure losses along a horizontal section by inserting a piece of pipe called as a stinger, having a smaller diameter than the liner.
Employing a reservoir simulator, the authors demonstrated the effectiveness of stinger application in improving the well productivity of thin oil zones sandwiched between a large gas cap and an active aquifer.