Effect Of Connate Water Research Articles

Effect of connate water on stress sensitivity of tight gas reservoirs

ABSTRACT The current research on stress sensitivity was mainly focused on reservoirs without connate water. But the connate water exists in tight gas reservoirs widely. Thereby research of stress sensitivity with connate water is meaningful for reasonable development of tight gas reservoirs. The micro-pore structure and clay minerals of core samples selected from tight gas reservoirs were analyzed through mercury injection experiment, X-ray diffraction analysis, and scanning electron microscope (SEM) technologies. For tight gas reservoirs, the clay content was high which is an important factor leading to strong stress sensitivity, and the characteristic of pore structure belonged to pore-matrix type. In view of the above, the mechanical capillary model considering connate water was established in this paper, which was used to analyze the influence of connate water on stress sensitivity. Core samples under different connate water saturations were adopted for the experiments of stress sensitivity. Results show that stress sensitivity of tight gas reservoirs with connate water is strong, and increases with decrease of permeability and increase of connate water saturation, which is consistent with the conclusion from the forced capillary model. Relationships between stress-sensitivity coefficients, absolute permeability, and connate water saturation were studied through experiments and a numeric expression used to quantificationally characterize stress sensitivity with connate water was obtained. On this basis, a productivity model considering stress sensitivity with connate water was established. By calculating the productivity of gas well of different connate water saturation, it can be found that productivity damage occurs due to stress sensitivity, and productivity damage increases with the increase of connate water saturation. Therefore, if the pressure drawdown rate is controlled and connate water saturation is decreased, productivity damage caused by stress sensitivity will decrease.

Inclusion of connate water in enhanced gas recovery reservoir simulations

Enhanced natural gas recovery (EGR) with supercritical (sc)CO2 sequestration offers the prospect of increased natural gas recovery. High-fidelity reservoir simulations offer a method to quantify the risk of contamination of produced gas by the injected scCO2. Simulations of scCO2 mixing with the reservoir gas have been reported; however the effects of connate water on EGR have not been effectively explored. We extend a prior EGR simulation tool (Patel, May and Johns, 2016; Ref. [1]) to incorporate connate water accounting for its effect on dispersivity and permeability; chemical equilibrium is modelled using a novel, computationally efficient Lagrange multiplier-based approach.The code is applied to a ‘quarter five-spot’ benchmark scenario. The inclusion of connate water generally resulted in a reduction in breakthrough time and a decrease in methane recovery. The connate water's largest effect was to change the scCO2 flow field, which sank towards the reservoir floor, flooded the lowermost accessible layers and entered the production well via a high throughput channel (‘coning’). The magnitude of these effects were, however, sensitive to well perforation depth, the influence of which was subsequently studied systematically. Well perforation depth was found to determine the duration of these sinking and coning events in a non-linear manner.

The Role of Connate Water Saturation in VAPEX Process

Abstract A large fraction of the physical model tests of the VAPEX process reported in the literature have been conducted without any connate water in the system. The absence of connate water was rationalized by suggesting that it has little or no influence on relative permeability of oil and since the vapourized solvent does not dissolve in water, there is no effect of water on the mass transfer process. However, this ignores the possible contribution of oil spreading at the gas-water interface to the mass transfer and the contribution of film drainage to oil relative permeability at low oil saturation. We have evaluated the effect of connate water on VAPEX performance using physical model experiments carried out in a visual model with different connate water saturations. Butane was used as the solvent and Ottawa sand was used for packing the model to obtain permeability and capillary pressure values comparable to field conditions. In addition to the visual observations of the size and shape of the vapour chamber, the rates of oil and gas production were monitored during the experiments. The results show that connate water has a measurable effect on the process, both in terms of the shape of the vapour chamber and the drainage rate of the diluted oil. The presence of connate water causes faster spreading of the vapour chamber in the lateral direction and tends to increase the thickness of the mixing zone. This increase in the mixing zone thickness appears to result from capillarity driven fingering phenomenon. The mixing zone had a distinct uneven appearance that was similar to patterns generated by frontal instabilities in miscible displacements. The effect of connate water on the drainage rate was an increase in the initial rate, but a reduction in the rate, subsequently. The presence of mobile water speeds up the communication between the two wells and leads to even faster spreading of the vapour chamber. Introduction Although several successful SAGD and CSS field projects have been reported, the high cost of steam generation, considerable energy loss, need for water and water treatment(1) are some of the problems which necessitate finding a more effective and environmentally friendly recovery method. Meanwhile, solvent-based processes, like VAPEX, have drawn more attention due to their potential advantages. In this process, vapourized light hydrocarbons, like propane and butane or a mixture of these with carrier gases, are used. VAPEX is a solvent analogue of the SAGD process and uses essentially the same well configuration(1). Solvent dissolves into the bitumen and reduces its viscosity through dilution, swelling and de-asphalting mechanisms(2). It has been noted that vapourized rather than liquid solvents produce a higher driving force for gravity drainage due to a higher density difference between bitumen and solvent(3). The optimum pressure for solvent injection is near its dew point pressure where the solvent solubility in the oil is high. The role of carrier gas is to keep the injected solvent in the vapour phase at the operating pressure.

98/03570 Effects of connate water on chemical flooding processes in porous media

98/03570 Effects of connate water on chemical flooding processes in porous media

Effects of connate water on immiscible displacement processes in porous media

Connate water is the name given to the water that occurs naturally in petroleum reservoirs in the form of an invisible dispersion or film within the oil phase. In this study, the effects of connate water on immiscible water/oil displacement processes in porous media were investigated. Special attention was directed towards the effects of connate water on viscous fingering instabilities and on oil recovery displacement efficiency. Experiments were performed using a two-dimensional, water-wet, consolidated porous medium cell of porosity 0.30. The fluid system employed was water displacing a lower-density paraffin oil phase. Three different flow modes were used for the experiments, namely, horizontal, vertical-upward and vertical-downward. For each flow mode, corresponding pairs of experiments were performed for five different flow rates in the absence and presence of connate water. At low flow rates, the irregularity of the viscous fingering pattern increased significantly in the presence of connate water and the corresponding oil recovery decreased. At high flow rates, the fingering pattern and oil recovery were largely independent of the flow mode and the presence of connate water. Depending on the flow mode situation, buoyancy effects can either stabilize or destabilize the displacement process when the densities of the two fluids are different.

Effect of Connate Water on Gas/Oil Relative Permeabilities for Water-Wet and Mixed-Wet Berea Rock

SUMMARY Gas and oil relative permeabilities were measured on water-wet and mixed-wet Berea sandstone at various connate (initial) water satura- tion values, including zero, using the gasflood method. The centrifuge method was used to extend the measured oil relative permeability curves to lower total liquid saturations than could be achieved by gasflood. Composite cores were used in the gasfloods, whereas individual plugs were tested in the centrifuge. The results show that gas and oil relative permeabilities are indepen- dent of the connate water saturation in water-wet and mixed-wet Berea sandstone provided certain conditions are satisfied.

The Effect of Connate Water on the Efficiency of High-Viscosity Waterfloods

Abstract High-viscosity water injection has been proposed for use in reservoirs containing high-viscosity crude oils. Previous publications have largely ignored the possible effects of the connate water on the proposed process. This paper describes experimental work which indicates that the connate water will be forced ahead of the injected water to form a bank of low-viscosity water. This decreases the oil recovery which would be expected if such a bank were not formed. These effects are shown for a range of fluid mobilities and connate-water saturations for a five-spot injection system. In general, oil recoveries using viscous water are significantly greater than for untreated water even though they are less than would be expected if no connate water bank were formed. Introduction The effect of mobility ratio on the oil recovery of waterfloods has been known for many years. Muskat first pointed out that the fluid mobilities (k/ mu) in the oil and water regions would affect the performance of the waterflood, and he estimated the general effect of these variables. Since this early work, studies of the effect of mobility ratio on secondary recovery have been reported where mathematical, potentiometric and scaled flow models were used. These studies have shown that a reduction in the mobility ratio between the oil and the displacing fluid would cause additional oil recovery when waterflooding reservoirs containing viscous crude oils. Studies reported by Pye and Sandiford have indicated that chemicals to increase injection water viscosity are now available and can be used to reduce the over-all mobility ratio of a waterflood. Where mobility ratios are controlled by the injection of viscous fluids, the connate water of the reservoir can play an important part in the displacement of the reservoir oil. The purpose of this study was to determine the effect of the connate-water saturation in waterfloods where viscous waters are used for injection. DISPLACEMENT OF THE CONNATE WATER Russell, Morgan and Muskat were the first to recognize the mobility of connate waters in waterflooding. They conducted waterfloods on oil-saturated cores containing 20 and 35 per cent irreducible water saturations, and found that from 80 to 90 per cent of the "irreducible" water was produced after only one pore volume of water was injected. However, their experiments were conducted at rates of flow significantly higher than those ordinarily occurring in waterfloods. Also, the cores were only from 4.0 to 8.5 cm long. Brown studied a 100-cm linear sand pack which had been prepared to contain connate water and oil. He used 140- and 1.8-cp oils with injection water of essentially the same viscosity as the connate water. He found that all of the connate water was displaced by the injection water in both cases. However, the injection volumes required for complete displacement of the connate water were considerably higher in the case of the more viscous oil. To verify the results of the foregoing experiment, a 10-ft-long linear model was constructed by packing 250–300 mesh sand in a 1/2-in. diameter nylon tube. The model was evacuated, saturated with a brine of 1-cp viscosity, and flooded with a 41-cp mineral oil to the irreducible water saturation of 10.9 per cent. The model was then waterflooded by the injection of a water solution which had an apparent viscosity of 42.6 cp. The solution consisted of 0.5 per cent methylcellulose in distilled water. The viscosities of the oil and connate water were measured with an Ostwald viscosimeter. The viscosity of the polymer solution was calculated by Darcy's law using pressures measured during actual flow conditions. The ratio of the mobility in the oil region to the mobility in the injection-water region was approximately 0.32. The mobility ratio of the oil region to the connate-water bank was approximately 14. The mobility ratio between the connate-water bank and the injection-water region was 0.024. Approximately 84.5 per cent of the recoverable oil was produced before water breakthrough. Immediately following breakthrough, oil and connate water were produced at an increasing water-oil ratio until the viscous injection water broke through. At viscous-water breakthrough, 96 per cent of the original connate water had been produced. After breakthrough of the viscous water, there was no additional production of connate water or oil. JPT P. 1481ˆ