SPE Advanced Technology Series

The paper summarizes the methods and effects of cyclic waterflooding in the southern oilfields of the Daqing Placanticline, discusses the feasibility and necessity of this process in lower permeability reservoirs, and analyzes systematically the main factors which impact the results of cyclic flooding. Production performance is compared and contrasted for fields developed by both conventional and cyclic waterflooding. Based on field case histories, the impact of cyclic waterflooding on performance at different water cuts is evaluated. It is demonstrated that cyclic flooding should be implemented at low water cuts, but is beneficial at all development stages. Cyclic waterflooding is an effective measure to enlarge sweep volume and enhance conformance efficiency and oil recovery. The practices described can provide guidance for cyclic waterflooding in similar oilfields.

In Daqing Oilfield, since 1988 1524 oil wells have been fractured by this technique. This is 21.4 percent of all fractured oil wells. The average oil production improvement of each well is 15.0 t/d, a 1.5 fold increase over conventionally fractured wells. The cumulative oil production improvement is 340.3x104 tons. 203 water wells were fractured by TMFUD, 33.0 percent of all the fractured water injection wells. The mean water injection improvement per day is 80.8 m3/d, and the cumulative water injection rate is increased by 146.6x104m3. The multi-fracture fracturing technique can be used to stimulate low permeability thin interlayers and as many as 6 to 9 independent sub-layers without moving the string.

Based on lab data this paper summarizes differences of petrophysical parameters of sandstones in Daqing Oil Field which were not watered out before waterflooding and were watered out after waterflooding. Pore throat dimensions, relative permeability, and wettability change with prolonged waterflooding. Using internal waterflooding at an early stage is a method of maintaining the reservoir energy and achieving long-term high and stable production. Injecting a great quantity of water for a long time is an effective means of enhancing oil recovery of reservoirs for Daqing Sandstone Oil Field.

From Darcy's law of two phases (oil and water), an analytic expression for oil and water vertical cross flow including gravity cross flow, capillary cross flow and additional cross flow under the effect of cyclic waterflooding is developed. Based on function J, this paper analyzes the vertical cross flow characteristics of capillary and its influence on production, explains the essential distinction between cyclic waterflooding and conventional waterflooding. Introduction

At the end of 1950's, a Former Soviet Union expert, M.R. Surkuchef, advanced the concept of cyclic waterflooding for the first time. He thought that production process could be improved by changing the type of water-injection and establishing non-steady state artificially in the reservoir. Based on this theory, the Former Soviet Union has conducted cyclic waterflooding in Pokorove oilfield and some other oil fields for field test and commercial production since 1964. All obtained better production results. The field test showed that cyclic waterflooding can improve recovery rate by 3 -10% compared with conventional waterflooding.

Since 1965, the Former Soviet Union focused on cyclic waterflooding theory research. Among those theories the most influential one was the mathematical model set up by O.I. Chenkowa et al. of the Former Soviet petroleum Scientific Institute in 1970's. The problem was that when cyclic waterflooding was conducted, this model could only predict the degree and influential factor of its improvement, by estimating the flow rate through porous medium between high and low permeability layers in vertical heterogeneous reservoir on the base of water stagnant coefficient concept; but could not explain the mechanism of cyclic waterflooding.

Through laboratory experiments, W.G. Ogengianiyanches et al. believes that the physical nature of improving production by cyclic waterflooding is that when injection is halted, capillary cross flow becomes important so more crude oil is displaced from layers with low permeability and the water saturation of high permeability watered-out layers is reduced. Thus, the conclusion that cylic waterflooding is more effective in water-wet reservoirs.

Field test and laboratory numerical simulation data show that cyclic waterflooding in oil-wet reservoirs can also be effective. This paper discusses the mechanism of cyclic waterflooding on the basis of seepage flow dynamics.

The Way of Oil & Water Vertical Cross Flow Under Cyclic Waterflooding

Set up a coordinate system, with the vertical downward direction as Z-axis. Suppose there are only two phases (oil and water) existing in the reservoir. Based on Darcy's Law, oil and water vertical movement equations can be written as:

(1)

(2)

Analyses of seepage flow and numerical simulation show that during the half cycle when injection is halted or reduced, another pressure differential between the high- and low-permeability layers besides capillary pressure and gravity potential is created. This kind of pressure differential inherent in unconventional water injection is called as additional pressure differential. In contrast, during the half cycle of reinjection or greater injection, the pressure buildup is quick in high permeability layers and slow in low permeability layers, creating a reverse pressure differential (Fig. 1). Then

(3)

Table (1) (2) and (3) as simultaneous equations and let

Then we have:

(4)

This paper describes development and applications of separate layer production technology, which facilitates full exploitation of the potential of each oil-bearing layer. The technology has evolved as water cut has increased in Daqing oilfield and focus has shifted to thinner, lower productivity layers. In Daqing field, 11 kinds of separate layer water injection strings, which can be divided into two main categories (hydraulic inflatable and compressional), 33 kinds of separate layer production (water shut off) strings divided into four main categories (including integral, slip support releasing, drill-able and pressure balance), four separate layer fracturing techniques (including sleeve separate layer, limited entry completion, multi-frac and positioning balance) have been developed.

Immiscible WAG injection was successfully used to enhance the oil recovery in the thick, heterogeneous reservoir in the east of Beier area of Daqing oil field. After a five year test period, the water cut is basically stable and production is continuously increasing. The recovery factor is now more than 3% higher than the ultimate recovery factor by water injection, and ultimate recovery is predicted to be more than 8% higher. Changes have taken place in the water entry profile of the injection well. Gravity segregation occurs in the reservoir. Areal sweep conditions are improved and the thickness of liquid production increased. Introduction

After water injection, pilot test and core data show that the highly heterogeneous thick reservoir with positive rhythm sedimentation is watered-out at the bottom. Ultimately, waterflooding waters-out 70% or so of the total thickness, so that remaining oil is mainly located in the upper part of reservoir. Recovering the remaining oil of this kind is very important in stabilizing the production of Daqing oil field.

Gas injection has been used as a method of secondary recovery to maintain reservoir pressure, but is limited by gas channeling because of the large oil/gas viscosity ratio. In the 1950s Caudle and Dyes suggested controlling gas channeling by WAG injection. The experimental study by Slack and Ehrlich shows that immiscible flooding in water-wet sandstone can reduce residual oil saturation by 18%. However, Emmett et al show that under a certain water saturation in oil-wet cores, water permeability in the presence of three phases is much lower than when only two phases (oil and water) are present. The mobility ratio can be improved by maintaining certain gas saturation in reservoir. Thus, the sweep volume can be extended. This is demonstrated by the 40% reduction in the water injectivity index of injection wells in the mixed-wet Jay oil reservoir (USA) after WAG injection. Champion and Shelden report that the water injectability in the mixed-wettability Kuparuk oilfield is decreased after immiscible WAG injection and predict an improvement in the waterflooding sweep efficiency. Stone notes the good vertical sweep of miscible WAG injection near injection wells. Far from the injection well, gravity segregation occurs and injected gas moves along the top of the reservoir. Many papers report only the study results of WAG injection, but not the change in the water injection profile and the liquid production profile after WAG injection. These papers only mentioned that mobility ratio can be improved by decreasing water injectivity, and concluded that WAG injection can improve waterflooding efficiency. There are no examples of using immiscible WAG injection as a tertiary production method in gently sloping (with 2 of inclination) heterogeneous thick reservoirs with positive rhythm sedimentation (permeability and grain size increasing with depth). A field test was performed in Daqing oilfield to verify the possibility of improving the recovery of this type of reservoir by WAG injection.

Physical simulation results before the test show that as a result of gravity segregation, injected gas moves along the top of the reservoir. When the top of the reservoir is plugged, the ultimate recovery factor increases by 6%. The same result is obtained from numerical simulation. When water gas ratio is 1:1 with alternating cycle of 1 month and annual injection rate of 6% of the pore volume, optimized development result are obtained. It is predicted that the ultimate recovery factor can increase by 6.4%. The WAG injection test commenced in March, 1989.

General condition of the test area

The WAG injection test area is located near the North 2-5-94 well in northeast part of Daqing oil field. Target SIII3-7 is the oil-prone sandstone layer, which consists of deltaic above water distributary deposits, which can be divided roughly into two depositional units (SIII3-4 SIII5-7). Medium to fine grained sands dominate the lower part of each unit. Fine siltstone and siltstone dominate the upper part. Argillaceous siltstone and silty mudstone dominate the top. Channel sandstone SIII5-7, has a sandstone thickness of 4.7m and a net thickness of 4.0m. The effective permeability is 370x10-3 m2. For SIII3-4, flood plain sedimentation developed in most areas except a channel sandstone developed in northern part.

Using the principle of fluid mechanics and the geometric properties of the annuli in directional wells, the law of laminar helical flow of drilling fluid in eccentric annuli was studied. Analytical solutions of apparent viscosity, velocities, flow rate, pressure drop and the stability parameter are presented. Results of numerical simulations are given. Laboratory studies and field application in eleven wells of Daqing oil field were used to validate the theory. A method for calculating pressure drop in annuli, which can be used in designing hydraulic parameters of directional wells is given.

The logging technology of radioisotopic carriers has long been a main means to monitor and survey injection profiles in during water injection. This paper focuses on the basic principles of the logging technology, radioisotopic microballs, logging tools, auxiliary equipment, logging techniques, data interpretation, and application effects.

Daqing Lamadian oil field is the largest oil field with a gas-cap in China. Monitoring, controlling and adjusting techniques have been used extensively to minimize cross flow. An inverted nine-spot pattern was used in the oil-bearing area. Gas-bearing layers were not perforated and there was a 400 to 600m buffer zone of oil and gas migration left between oil- and gas-bearing areas. A monitoring system for gas-cap performance was established. Flow between the oil-bearing and gas-bearing layers has been successfully controlled and a relatively stable oil-gas interface has been maintained for 20 years.

The main reservoir of the Daqing field, the PI reservoir, was evaluated after total water cut had increased to about 90%. The location of high residual oil saturation was correlated with the depositional environment. Recompletion and redevelopment plans were formulated for each lithology. Wells were selectively perforated and additional infill wells were drilled. Finally, a polymer flood was initiated. It was concluded that the reservoir should be perforated twice to increase the volume swept by waterflood before injecting polymer.