Panteha Ghahri

A Thorough Investigation of Clean-up Efficiency of Hydraulic Fractured Wells Using Statistical Approaches

Hydraulic fracturing is considered as one of the most effective stimulation techniques to improve recovery especially from unconventional low permeability reservoirs. However this promising stimulation technique sometimes does not respond as expected. Significant amount of work has been dedicated to this topic with ineffective fracturing fluid (FF) clean-up considered as one of the main reasons for this underperformance. However there are still great deals of uncertainties in this area primarily due to large number of parameters affecting FF invasion and its back flow. This work presents results of 10 different sets of numerical simulations consisting of injection, soaking and production periods for 40960 runs. Each set consists of 4096 runs and investigates the simultaneous impact of 12 pertinent parameters (fracture permeability, matrix permeability (km), end points and exponents of Corey gas and FF relative permeability curves in both matrix and fracture and matrix capillary pressure (Pcm) (depending on interfacial tension (IFT), km and pore size index). Two-level full factorial experimental design and linear response surface statistical approaches were used to sample the variables domain, covering a wide practical range determined with the support of our 11 industrial sponsors, and generate output response. Results indicate that improvement in FF mobility inside the fracture is the major factor affecting FF cleanup efficiency. In line with this finding, maintaining high Pcm, by retaining high IFT, results in cleaner fracture (lower gas production loss, GPL). That is, increasing IFT retains FF within the matrix and allows more gas to flow freely inside the fracture. This was confirmed by the corresponding saturation map of FF distribution. The effect of Pcm was more pronounced when drawdown was very low and/or soaking time was extended. At very low drawdown and when km was reduced, in a set within its variation range, the effect of the resultant increase of Pcm on GPL was more pronounced than that of the resultant decrease in FF mobility. Generally when injected FF volume increased, larger GPL was observed and reduction of GPL (cleanup) was also slower. As fracture length decreased, cleanup was faster and the effect of fracture pertinent parameters on GPL, compared to those of matrix, decreased. This works findings allows better evaluation of benefits of this costly operation leading to an optimized design and more efficient ways to improve its performance. For instance, sometimes aiming for a longer fracture, due to its FF poor cleanup performance, is not practically attractive and use of IFT reducing agents to produce more FF during the back flow period, would not have the intended impact to bring back its performance to the desired ideal level. Copyright © 2014, Society of Petroleum Engineers.

A Study of Hydraulic Fracturing Clean-up Efficiency in Unconventional Gas Reservoirs Using Statistical Approaches

Hydraulic fracturing is widely used to improve well productivity especially in unconventional reservoirs. This costly operation, however, sometimes underperforms. One of the main reasons for this poor performance is poor clean-up efficiency of injected fracturing fluid (FF). In this work, a parametric study of FF clean-up efficiency of hydraulic fractured vertical wells was performed with 49152 simulations (in 12 sets) consisting of injection, soaking and production periods. Due to the large number of required simulations, that were conducted using a commercial reservoir simulator, a developed computer code was used to automatically read input data, run simulations and creates output data. In each set (consisting of 4096 runs), simultaneous impacts of 12 parameters (fracture permeability, matrix permeability and capillary pressure, end points and exponents of Corey gas and FF relative permeability curve in both matrix and fracture)were studied. To sample the variables domain and analyse results, two-level full factorial experimental design and linear surface model describing dependency of gas production loss (GPL), compared to 100% clean-up, to pertinent parameters at three production periods (10, 30 and 365 days) were considered and supported by the tornado charts of fitted equations, frequency of simulations with given GPL and FF saturation maps. Results indicate that generally parameters controlling FF mobility within fracture had greatest impact on GPL reduction. However in sets with very low matrix permeability especially when applied pressure drop during production is low, the effect of fluid mobility in the matrix on GPL is more pronounced, in other words, it is important how gas and FF flow within matrix rather than how fast fracture is cleaned. In tighter gas formations, generally more GPL and slower clean-up was observed. The effect of matrix capillary pressure on GPL reduction was more pronounced when drawdown was very low and/or soaking time was extended. This observation was more profound in tighter formations, i.e. for these formations, the effect of a change in drawdown and/or soaking time on matrix capillary pressure and GPL was more pronounced. These findings can be used to make better decisions on the performance and optimised design of hydraulic fracturing, which is a costly but widely used stimulation technique for unconventional low permeability gas reservoirs.