No abstract

A study was undertaken with the objective of evaluating the use of low concentrations of surfactant to improve oil recovery. Various concentrations of commercial surfactants were screened for long term stability at high temperature in seawater. Cloud points, precipitation, and the stability of surface tension were used as screening criteria. Spontaneous imbibition tests at ambient and reservoir temperatures were conducted using reservoir chalk plugs that were moderately water-wet. A selected surfactant, when added at low concentrations (100 to 500 parts per million of active surfactant) to the imbibition water reduced the residual oil saturation over imbibition tests conducted without surfactant. Acceleration of spontaneous imbibition was observed in tests that had improved oil recovery. Forced imbibition tests for viscous displacement were conducted by flow tests. Although low concentrations of surfactant did not lower oil-water interfacial tension below single digits, a reduction in residual oil saturation was obtained in the forced imbibition tests over tests without surfactant. Measurement of surfactant adsorption indicated that low adsorption at reservoir conditions could be obtained below the critical micelle concentration (CMC) of some surfactants. The combination of improved recovery with low adsorption suggests that the addition of surfactant to injected water may improve the economics of surfactant-enhanced water flooding under appropriate conditions. Introduction Surfactants have long been considered for improving oil recovery from oil reservoirs. Although many pilot tests and a few field tests have been conducted, the economics of surfactant injection have rarely been favorable. Most surfactant studies focused on the injection of a high concentration of surfactant to create ultralow interfacial tension between oil and water via microemulsions followed by a slug of polymer designed to mobilize the surfactant bank and oil. The front end cost of such a process was only favorable if residual oil saturations could be reduced to near zero, surfactant adsorption was not excessive, the sweep efficiency was excellent and oil prices were high. In contrast, consideration herein was given to improving the economics by a process that used only a dilute amount of surfactant to minimize adsorption and to recover only a portion of the residual oil. Published field tests1,2 show that it is possible to have an economic surfactant process at low surfactant concentrations. Others have examined in the laboratory, the possibility of dilute surfactant flooding for improving oil recovery3,4,5. This paper will focus on improvements observed in the displacement of oil in the laboratory and the approach to minimize surfactant losses. Description and Application of Equipment and Processes Preparation of Materials. Kansas outcrop chalk and field reservoir chalk were used in this study. Field reservoir plugs were extracted by alternation of toluene and methanol soaks until no hydrocarbon discoloration was observed (1–2 months). All plugs were one inch in diameter and up to 3.5 inches in length. The wett ability of the Kansas outcrop plugs was altered by aging under field crude oil at initial water saturation using established methods6 and then extracted like the field reservoir plugs. Spontaneous imbibition tests were used to establish a baseline measurement and to select moderately water-wet plugs for further testing. The field stock-tank crude oil was centrifuged to remove paraffins that were solid at near-ambient temperatures. N-decane of 99+% purity was used and filtered through a silica gel column before use. The brines were equilibrated with chalk material and filtered before use. Other fluids were used as purchased. Surfactants were commercially available products. They were diluted with synthetic North Sea water to desired concentrations before use. Preparation of Materials. Kansas outcrop chalk and field reservoir chalk were used in this study. Field reservoir plugs were extracted by alternation of toluene and methanol soaks until no hydrocarbon discoloration was observed (1–2 months). All plugs were one inch in diameter and up to 3.5 inches in length. The wett ability of the Kansas outcrop plugs was altered by aging under field crude oil at initial water saturation using established methods6 and then extracted like the field reservoir plugs. Spontaneous imbibition tests were used to establish a baseline measurement and to select moderately water-wet plugs for further testing. The field stock-tank crude oil was centrifuged to remove paraffins that were solid at near-ambient temperatures. N-decane of 99+% purity was used and filtered through a silica gel column before use. The brines were equilibrated with chalk material and filtered before use. Other fluids were used as purchased. Surfactants were commercially available products. They were diluted with synthetic North Sea water to desired concentrations before use.

The effect of embedded fractures on the movement and recovery of hydrocarbon from larger outcrop chalk blocks at different wettabilities has been measured in the laboratory. 2-D nuclear tracer imaging was used to produce in-situ fluid saturation distributions during oil production. Emphasis was on determining the oil recovery mechanisms by tracking the flow path of the advancing water. Two sequential waterfloods were performed on each of the three different blocks: first before fracturing, and then after fracturing. The same fracture network configuration was used for all three blocks, which were strongly water wet, moderately water wet and near neutral water wet. Waterflooding of the unfractured blocks, at high initial water saturation, occurred with minimal water banking while waterflooding at low initial water saturation produced distinct water bank formation. Waterflooding of the fractured blocks showed that the "closed" fractures produced a significant effect on fluid movement in the strongly water wet block, but only minor effect for the moderately and near neutral water wet blocks. The open fracture affected flow in all the blocks. Total oil recovery was higher in the strongly water wet block than in the moderately water wet block with the lowest oil recovery observed in the near neutral wet block. Introduction In fractured chalk reservoirs it is generally believed that oil production results from spontaneous imbibition of water from the fracture network and subsequent movement of the expelled oil through the fractures to the producing wells. However. if there were a significant amount of capillary continuity between adjacent blocks, viscous displacement of oil could also play a role and the matrix pore network could provide an alternate path for oil movement toward the production wells. Viscous displacement in a fractured chalk should be most important near water injection wells, during waterfloods and in reservoirs which are less than strongly water-wet. Previous experimental work which are pertinent to the assessment of fluid flow in fractured chalk include:The monitoring of saturation distribution during spontaneous axial imbibition in stacked cores using both horizontal and vertical configurations.Measuring saturation distributions as a function of time for the spontaneous imbibition and waterflooding of cores of different length and the area and configuration of exposed faces.The saturation distributions produced by gravitational drainage andfree gas. 2-dimensional saturation distributions have been monitored in larger strongly water-wet chalk blocks with several fracture orientations. Techniques have been developed which reproducibly alter the wettability of outcrop chalk to mimic the less than strongly water-wet chalk in a reservoir. This study was conducted to follow fluid movement and investigate hydrocarbon recovery mechanisms in fractured chalk at less than strongly water-wet conditions. The fracture network for these larger chalk blocks is similar to previously reported experiments on strongly water-wet fractured chalk. Experimental Three blocks, approximately 20 cm × 12 cm × 5 cm thick, were cut from large pieces of Roerdaloutcrop chalk obtained from the Portland quarry near Alborg, Denmark. This chalk material had never been contacted by oil and was strongly water-wet. The blocks were oven dried for three days at 90 C, end pieces were mounted and the whole assembly was epoxy coated. Each end piece contained three fittings so that entering and exiting fluids were evenly distributed with respect to height. The blocks were vacuum evacuated and saturated with brine containing 5 wt% NaCl + 5 wt% CaCl2. Porosity was determined from weight measurements and the permeability was measured across the epoxy coated blocks, see Table 2. P. 559^

No abstract

The effects of fractures on oil recovery and in-situ saturation development in fractured chalk blocks have been determined for several representative wettabilities. These effects were visualized using two complimentary two-dimensional in-situ imaging techniques; nuclear tracer imaging (NTI) for the experiments with large blocks of chalk and magnetic resonance imaging (MRI) for high spatial resolution to visualize fluid flow patterns inside fractures between two stacked core plugs. For three different wettabilities, the specific patterns of saturation development were monitored with NTI and the mechanisms of fracture crossing were determined using MRI.