Capillary pressure phenomena in the vicinity of the wellbore govern the rates of production of oil and water because the relative fluid saturations are functions of the capillary pressure existing between the fluid phases. Assuming that water is the wetting phase in contact with the porous system, which is phase in contact with the porous system, which is generally true of fields that have been waterflooded, the imbibition capillary pressure curve applies since the water saturation at the well is gradually increasing. With water wetting the solid, the capillary pressure of the aqueous phase must be continuous pressure of the aqueous phase must be continuous within the porous system and into the well. Capillary pressure does not exist (is zero) at the exit face pressure does not exist (is zero) at the exit face with the rock (the wellbore), therefore water cannot flow into the well until a condition of zero capillary pressure is obtained within the formation in the pressure is obtained within the formation in the vicinity of the well. Hence, a newly drilled well initially produces only oil until the capillary pressure reduces to zero. Water is then produced in pressure reduces to zero. Water is then produced in greater and greater quantities as the water saturation moves from the point of zero capillary pressure toward a residual oil saturation by imbibition. When imbibition ceases the imbibition capillary pressure curve becomes negative requiring input of pressure curve becomes negative requiring input of greater energy for simultaneous production of oil and water, until finally the imbibition curve approaches a negatively infinite value and all production of oil stops as the residual oil saturation is attained.
The pore size distribution of the rock governs the capillary pressure behavior. A decrease of the pore size distribution produces a higher residual oil pore size distribution produces a higher residual oil saturation and results in a decrease of the oil production rate that may be as much as 2-5 times less. production rate that may be as much as 2-5 times less. Factors adversely changing the pore size distribution, and hence the capillary pressure, at the production well are migration of fines, precipitation of production well are migration of fines, precipitation of crude oil components (paraffins and asphaltenes), and campaction of the sand due to pressure draw-down.
These aspects of adverse capillary phenomena at the wellbore are analyzed by capillary pressure measurements of cores having large and small pore sizes and by comparison to results with cores which are partially plugged with fines, bacteria, etc. The pore size distributions and capillary pressure are measured before and after plugging. pressure are measured before and after plugging
Many of the oilfields in the United States are rapidly approaching abandonment because oil production rates have declined to the point where economic operation of the oilfield is no longer feasible. High water/oil production ratios are attributed to attainment of a residual oil saturation in the reservoir; however, in many cases, the migration of fines to the immediate vicinity of the wellbore and precipitation of paraffins and asphaltenes may have decreased the effective pore size distribution near the well. Reduction of the effective pore size distribution produces capillary pressure changes that lead to a premature high water/oil production ratio.
This paper reviews the basic capillary pressure phenomena in porous media applied to production phenomena in porous media applied to production wells and presents experimental verification of the large adverse change of capillary pressure that occurs in rocks due to reduction of the effective pore size distribution. The effects of these pore size distribution. The effects of these changes on production wells is analyzed and methods for possible improvement are suggested.
PROCEDURES PROCEDURES This analysis of one aspect of production well behavior is based on capillary pressure curves of rocks saturated with brine and crude oil. Considerable practical and theoretical knowledge of the capillary phenomena of water-oil-rock systems is required for its application to the analysis of production well performance. production well performance.