At ESal, we strive to provide much faster and less expensive analysis of reservoir wettability, as well as an assessment of the potential to control wettability by salinity, versus the industry average of 4-6 years and millions of dollars. Our business model means we can provide similar results in a fraction of the normal time and expense. The most desirable outcome of a project is to assess and alter the in-situ wettability of the candidate petroleum reservoir to increase oil recovery, however even knowing that reservoir wettability is insensitive to salinity is valuable information for operations. We deliver this information to you by evaluating the relationship between salinity and wettability for the target reservoir.
There are three main stages in the workflow: screening, laboratory testing, and deployment. In order to provide an initial assessment, we developed an analytical screening tool based on hundreds of field and laboratory examples. Not all reservoirs have wettability that is sensitive to salinity and our screening tool saves time and money by eliminating marginal candidates. After favorable screening results are observed, laboratory testing quantifies the current reservoir wettability as well as the salinity sensitivity. Finally, the laboratory data is used to develop economic evaluations and serve as the basis to formulate chemical specifications for the injected water.
In the first step, we evaluate if the process is likely to be effective at your site. Screening refers to the technique of using a set of rules or mathematical relationships to evaluate the candidate projects. Screening is semi-quantitative and provides a first look at suitability of the candidate property for wettability alteration by salinity control. A good score means the project has favorable characteristics and should proceed to the next step, laboratory testing, to quantify the benefit.
If the result of screening is positive, we recommend that the client proceed to laboratory testing to verify that the wettability is sensitive to salinity and how much the wettability can be altered. The process of altering salinity to improve wettability will not work in all fields, and it is important to avoid projects with a low chance of success before substantial investment. Our screening tool incorporates all the public data for successes and failures worldwide and calculates a score based on the key factors that determined those outcomes. For instance, there can be a strong positive correlation between clay content and incremental recovery as seen in this graph.
Input data include up to 22 parameters such as production history and oil composition, reservoir water salinity and chemistry, and rock mineralogy (XRD, thin section analysis, reports) porosity and permeability of pay intervals, net pay, and thickness of pay intervals (if there are multiple intervals).
The ESal Screening Tool™ produces a numeric scoring between 0 and 100 with 100 being the best possible potential. Screening may be done for a single location, several fields, or formations across an entire basin. The screening process is depicted below and shows some of the empirical relationships used and example scores for 98 Wyoming oil fields (bar chart) and the Queen Sandstone (map view) in the Permian Basin. In the Wyoming case, we would recommend laboratory testing proceed for the three excellent and six good fields to quantitatively assess the response of wettability to salinity.
In the second step, the relationship between salinity and wettability for the candidate reservoir is determined by laboratory testing using a modified flotation technique (MFT). This method uses the oil and rock from the candidate reservoir with produced water or synthetic brine (based on produced water chemistry). MFT is a flotation technique where water and oil are added to powdered rock, mixed and allowed to physically separate as illustrated below. The wetting of the solid grains is controlled by the balance of surface forces between the oil and water with the grain surfaces.
All grains start out as water-wet and are negatively buoyant. Oil is added and the rock and oil phases are thoroughly mixed. After equilibration, the grains physically separate, with water-wet grains sinking and oil-wet grains remaining suspended in the oil-phase. We can rapidly determine the wettability of the reservoir system and test the wettability across a range of salinity. This procedure makes it practical to perform numerous measurements to evaluate the relationship between salinity and wettability for candidate reservoirs.
Results from a typical salinity series is shown below. The figure (right) shows the response of wettability to salinity for two shale reservoirs in the Permian Basin with initial salinity on the left and salinity declining to the right. Both reservoirs are initially strongly water-wet. In one case, the reservoir wettability is insensitive to salinity, however in the second reservoir, the initial water-wet condition can be altered to neutral-wet, improving recovery.
The graph (left) shows another example in offshore chalk where the reservoir system is initially oil-wet. As salinity is lowered, the system becomes almost neutral wet.
The final step in the ESal workflow is deployment. The below figure depicts the inputs for this step and the final product – the water chemistry specifications. During this step, ESal uses the laboratory testing results as input for economic evaluations and to guide design of the injection salinity. In the water chemistry design phase, we use geochemical modeling to account for the reactions between the injection water, reservoir minerals and reservoir water. Injected chemistry is also designed to avoid any formation damage from precipitation, clay mobility and swelling. This information is used to develop injection water salinity specifications and deployment options.
The injection water can be created by mixing the produced water with a local water source, some combination of sources, or by treatment of the produced water. We assess the local water resources and available treatment options, both cost and volumes, to provide inputs to the economic evaluation. The figure below shows the measured change in wettability on the left-hand graph and the resulting shift in residual saturation projected onto the drainage curve (shaded curve in the center graph). In this example, the shift in wettability will increase recovery by about 0.2 saturation units.
This figure also depicts the final step – the economic evaluation and projected cash flow projections. We can see the laboratory data shows that maximum benefit (shift to neutral wettability) occurs over a range of salinity (shaded zone). Therefore, the injected water salinity has an effect over a broad range of salinity, making operations potentially less expensive. The economic modeling includes Monte Carlo analysis of this projected benefit and uses conservative assumptions to provide realistic projections. Using data from industry leaders and comprehensive analysis, multiple scenarios are created for every individual project to evaluate potential risks, costs, and benefits. Finally, ESal will provide field support to ensure the operations personnel understand how to achieve the best outcome and to continually analyze project results to see if we can further improve the injected water chemistry.