Effect of wave action on near-well zone cleaning

Drilling filtrate invasion into the producing formation and native water accumulating of the near-well zone in well operation reduce the well productivity. As a result of that, depending on characteristic capillary pressure scale and differential pressure drawdown, oil production rate may become lower than expected one. In this paper, it is considered the hysteresis effects of capillary pressure after reversion of displacement. As applied to laboratory experiment conditions, the solution of problem of oil flow in formation model with a pressure drop on the model sides harmonically varied with time is presented. It was estimated a range of fluid vibration effective action on the near-well zone cleaning from capillary locking water. The plant simulating extraction of oil from formation using widely practised sucker-rod pump has been created. Formation model is presented as a slot filled with broken glass between two plates. In the process, natural oil and sodium chloride solution were used as working fluids. The experiments qualitatively confirm a positive effect of jack pumps on the near-well zone cleaning.


Introduction. Flow rate of producing well
Wells drill with use of a water clay solution, with consequent water filtrate invasion into the producing formation. Thereby, an oil or gas saturation of near-well zone reduces. When influx is made with a well bore pressure being less than initial strata pressure , the penetrated water filtrate is c p p  0 p p  not fully displaced. Part of water remains in porous medium in an immiscible capillary-locking state.
In some cases, a stratal flow characteristic change near borehole is said to be skin effect [1]. Let us consider axisymmetric steady oil influx to well in a stratum with initial oil saturation . 0 s s  Suppose that the stratum is hydrophilic, so some water is capillary locked in the near-well zone. We denote as coordinate of external boundary where and . Let such boundary exist.
So the Muscat-Leverett system of equations for steady flow takes the form: Here is the capillary pressure, the functions and are specified on interval (0,1) and . At steady regime of capillary locking the relation is satisfied. In this case, , and for oil flow velocity we have the formula Substituting this into the continuity equation for oil phase, we obtain the well production expression Here, and, if we take the approximation , , this integral can be expressed in terms of elementary functions: If the capillary forces are negligible small (in this case water is also movable), so flow of fluids is homogeneous, , and for the oil influx we obtain the expression As can be seen, due to partial plugging of the near-well zone by water, the well production decreases in times, where  .

Mixed wettable media. Reversion of displacement
Appearing in the formula for capillary pressure jump on phase boundary, parameter defines a 0 k p degree of the rock wettability with water. Positive values of the parameter are typical for hydrophilic medium and negative ones are associated with hydrophobic rock. Most of oil-bearing rocks are mixed wettable. In such strata, pressures in different phases become equal in some inner points . A (0;1) s   sign of parameter can change while immiscible displacement changes direction. As an illustration, 0 k p in Figure 2 is presented the photo of oil inclusions in the water-filled quartz capillary (radius m) [2]. Experiments show that a convexity of meniscuses changes after a change of 3 0,3 10 k r    flow velocity direction. Capillary forces impede the displacement process because the energy of particle separation from capillary surface is in excess of the energy released in the course of its adhesion to the same surface. Under change of displacement regime or its direction in the formations saturated with oil and water, a capillary pressure is subjected to the hysteresis phenomenon. A way of plotting of hysteresis loops of capillary pressure is described in paper [3].

Pressure wave propagation against the background of steady-state flow
Applying to laboratory experiment conditions, we consider one-dimensional oil flow in finite-size pattern with specified pressure drop on the sides and harmonically varied with time 0 c p p p    pressure at the input. The process is described by problem: