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Maximum oil rate is achieved by maintaining formation in such a mode, as to minimize negative effects in the bottomhole area, which appear due to free gas, which separates from oil, blocking oil flow, and due to forming of zones of sluggish, viscous degassed oil. Application of this method also decelerates pressure drop in formation by lessening premature exiting of gas from formation, and decreases current value of GOR. This leads to extended life of well and increased ultimate production index.
The method applies to all oil production methods, such as fountain, gas-lift and pump, and allows receiving millions of additional barrels of oil without drilling any extra wells or building any additional expensive platforms for offshore wells.
Example of Method Utilization
Let’s examine a sample application of this method, using mathematical model of processes describing the major mechanisms appearing in formation during non-stationary filtration of two-phase fluid. For that let’s employ one-dimensional axis-symmetrical system of Muskat equations , with corresponding PVT characteristics of fluid and dependencies of relative permeability Kro, Krg from liquid saturation (SL):
Where: P(r,t) – pressure in formation; So(r,t) – oil saturation in formation; Sg(r,t) – gas saturation in formation; Rs(P) – solution of gas in oil; Bo(P) – oil formation volume factor; Bg(P) – gas formation volume factor; MUo(P) – oil viscosity; MUg(P) – oil viscosity; FI - formation porosity; K – formation permeability.
There was created a corresponding simulator for purposes of analysis and prediction. For example, the following formation was analyzed: radius Rf=1000 ft; height H=50 ft; FI=0.15; K=15 milliD, rw=0.3 ft, with PVT characteristics shown on fig.3 and functions Kro(SL) and Krg(SL) shown on fig.4. The resulting three cases of solution are shown on fig.5: I – the case when bottomhole pressure was kept at Pbot(t)=0.25*Pf(t); II – the case when bottomhole pressure was kept at Pbot(t)=Pbotopt(t); III – the case when at first for approximately 120 days the well worked according to scenario I, and then it was switched to scenario II.
Behaviors of oil rate (Qoil), formation pressure (Pf), and GOR, in dependence of current recovery index (N) are shown on fig.1. In case I, the well worked for approximately 990 days before the oil rate fell to 6 bar/day, the limit of production sensibility. By that time the well gave ~4.25% ultimate recovery index. In the second case the well worked for 1440 days, and gave ~9.8% ultimate recovery index (more than double!). In case III, when the well was switched to optimal regime 120 days after production started, the ultimate oil recovery index increased from 4.25% to ~6.2%. At the same time, switching the well into optimal regime reduced GOR and increased oil rate from 130 bar/day to 250 bar/day.
All these positive effects were achieved due to keeping bottom hole pressure at the optimal level, which brought to reduction of forming of oil blocking zone in formation near bottomhole and slowed down loss of gas from formation, which causes formation pressure to drop.
Advantages of This Method
Maintaining optimal bottom hole pressure allows to:1. Increase current oil rate.
2. Reduce current GOR and WOR.
3. Avoid premature loss of formation pressure and energy.
4. Eliminate appearance of high viscosity areas in formation near bottom hole zone.
5. Increase formation’s relative permeability coefficient by oil.
6. Increase productivity index of the formation.
7. Increase recovery efficiency of the well and the oil field.