Casing optimization through predictive analysis optimizes frac efficiency
When speaking with completion engineers about their priorities in planning and executing a hydraulic fracturing job, efficiency and cost sit near the top of their list. These aspects are so closely intertwined and co-dependent that one cannot be manipulated without carefully considering the effect of the other. When considering the frac job holistically, many factors can be tweaked to maximize efficiency while also reducing costs. Two of these factors are pumping rate and pressure.
Rate and pressure are the foundation of a frac job. Drilling programs, casing designs, frac horsepower needs and many other factors rely solely on rate and pressure requirements for a well, with the objective that one can reach top rate throughout the frac while remaining under the maximum allowable surface pressure. Both rate and pressure are chosen in ways to enhance the efficiency of the job and optimize stimulation in efforts to maximize the estimated ultimate recovery of the well.
Identifying the challenge. So, what happens when one of these requirements cannot be reached during the frac? In most cases, the shortfall comes in reduced treatment rates, due to high treatment pressures. These diminished rates can drastically affect the efficiency of the frac job, consequentially driving up overall completion costs and delaying cash flow returns. With a high-performance friction reducer already being used, and limited desire to make drastic changes to the perforation or sand slurry design, options are limited to counter high pressure and reduced treatment rates. If similar problems are predicted on future wells, an adjustment in some aspect of the completion design must be made early in the planning process.
Analytical approach. To help operators solve this issue, Universal Pressure Pumping has developed a method for evaluating casing design sensitivity, using a predictive pressure analysis tool. The internally developed tool provides the ability to input planned pre-job parameters, such as fluid type, perforation design and fracture gradients, which generate a summary of the expected treatment.
Some of the most important outputs include tubing friction, maximum achievable rate and pressure at maximum rate, all on a stage-by-stage basis. Knowing these factors, the tech services team has the ability to present variations in casing design that can positively affect the overall treatment and frac efficiency of the well. By providing operators with an extensive breakdown differentiating potential casing designs and the effects of those designs on their wells, they can confidently choose a design that best aligns with their desired well treatment, costs and efficiencies.
Many factors are considered when using the predictive pressure analysis tool. Most inputs are design constants, including well lengths and depths, stage count and treatment volumes. The tool though, also includes factors that have more potential for variability like perforation design, estimated average instantaneous shutdown pressure (ISDP), percent friction reduction and, of course, casing design. These are the variables that are explored most by the technical services team when looking at ways to maximize rate throughout a job.
A few of these variable inputs are relatively easy to investigate and are identified as “educated constants,” meaning that though they are not operator-provided constants, like measured depth or stage count, they still could be accurately estimated for the pressure analysis. These educated constants include perforation design, average ISDP and percent friction reduction. Although modification in perforation and cluster design influence rate and pressure, without a drastic change, the alterations yield is negligible to overall treatment. Estimating average ISDP is performed by gathering and analyzing completion data from nearby wells in similar plays, and then predictions are made based on comparable information.
Pre-job water analysis utilizing water that will be used on the job and a variety of friction reducers at the operator’s disposal provides an accurate friction reduction percentage for the analysis. With the number of potential variable factors now reduced to one, casing optimization can now be the focus. Casing design changes provide the largest range in rate and pressure fluctuations from the well. Many factors are taken into consideration when analyzing a casing redesign for a more effective frac, including single string vs. tapered string, casing size and casing weight.
Table 1 illustrates a generalized example of the wide range of maximum rates and pressures of a well, based on its casing design. Some of the major sources of rate and pressure differences are tubing friction and the maximum pressure rating of the casing. Changing casing size, or introducing a tapered string to the design, has a significant effect on total tubing friction throughout the system. Tubing friction has a direct correlation to surface pressure, which in turn directly affects the achievable rate. Weight of the casing used throughout the design also must be closely considered. Increasing the casing weight of the weakest part of the string will up the maximum pressure rating of the total design, in turn increasing the achievable rate of the well.
Operators can consider this comparative analysis and explore options, based on their anticipated completion design, cost and casing availability. Casing design B and C yield similar results in terms of rate and pressure analysis, but may include a string of casing that is unavailable or uneconomical to purchase, Table 1. Through this analysis, an operator also may conclude that if they increase casing size, they may be able to extend the length of well lateral, due to reduction in friction pressure when compared with the original design. These types of scenarios, and many others, must be taken into consideration by the operator when designing their casing string. The predictive pressure analysis tool is particularly useful here, because of the ability to quickly change inputs and rapidly sort through the multitude of viable and nonviable options.
The ease and accuracy of the predictive analysis tool recently led a Permian-based operator to seek advice from the technical services team regarding pressure analysis confirmation. Working in a sensitive formation, where operating at top rate was key to optimizing fracture complexity, the customer wanted to confirm that the current casing design would be able to withstand a 20% increase in rate while allowing them to remain under the maximum pressure rating of the well. Using current well parameters and pre-frac inputs, the team was able to confirm that the well could, indeed, be fractured at optimum rates while staying below maximum pressure restraints throughout the entire job.
Figure 1 represents a stage-by-stage analysis of the actual job treatment, paralleled with the pre-frac predictive pressure analysis, indicating the frac job held true to predictive analysis, and the job was completed without any rate or pressure limitations. Not only was the operator able to increase the rate of stimulation for a more effective frac, but there were associated benefits of a more efficient frac timeline. By decreasing the overall time on pad by 20%, the operator was able to effectively bring production forward, resulting in an earlier time to cash. The operator also decreased associated costs, such as operational day rates and equipment rentals, by effectively shortening their time on pad while optimally completing the job.
The long game is efficiency and cost. If an operator can use predictive analysis to develop or analyze a casing string that allows them to reach maximum rates while remaining under top pressure restrictions, then frac efficiency becomes a less volatile factor. With the potential for delays and setbacks now reduced as a result of a more effective casing design, operators can more accurately develop operational timelines and look at other aspects of the completion process for cost savings.
It is also important to recognize that an unplanned rate reduction not only affects frac efficiencies, but it also could affect well production. Verifying that perforation velocities remain in the target range for maximum EUR is imperative to the well’s success. Using the predictive pressure analysis tool with a focus on casing optimization allows operators to accurately forecast timelines, costs and efficiencies while knowing there will be little volatility in the design they choose.
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