Techniques for optimized completions shown to result in higher EURs, flattened decline curves
By Kelli Ainsworth Robinson, Associate Editor
Optimized completion techniques – including longer laterals, tighter cluster spacing, and higher proppant loads and stage counts – can significantly increase the number of conductive transverse fractures and the estimated ultimate recovery (EUR) of Haynesville wells. The finding was made by Halliburton and Dallas-based E&P company Covey Park Energy in a study comparing two Haynesville wells – one completed in 2010 and one in 2016. Additionally, the study found that modern completion techniques can flatten the decline curve in Haynesville wells.
The evolution of completion designs in the Haynesville have been driven, in large part, by the play’s low reservoir permeability, R.J. Cadotte, Technical Sales Advisor for Halliburton, said during a presentation at the 2017 SPE Annual Technical Conference and Exhibition in San Antonio, Texas, in October. “Any part of the reservoir that’s not contacted with a fracture won’t help increase the EUR of that well,” he said.
Because of the play’s low reservoir permeability, completions in the Haynesville are typically designed to create as many conductive transverse fractures as possible. However, this goal is challenged by the fact that the Haynesville is somewhat softer and more ductile than other North American shale plays. This softness, combined with a high closure stress gradient, makes wells in this region prone to proppant embedment. It also leaves fractures susceptible to losing width or even closing in areas where proppant concentration is low. To overcome these challenges and optimize production, operators have been increasing the proppant load and stage counts per well, while drilling longer laterals and decreasing cluster spacing, Mr Cadotte said.
In 2011, the amount of proppant per lateral foot in the majority of Haynesville completions ranged from 750 to 1,500 lb/ft. The following year, proppant volumes rose to an average 2,500 lb/ft and held steady until 2016, when volumes rose again to an average of 3,000 lb/ft. In 2017, proppant volumes per lateral foot jumped to between 3,500 and 5,000 lb/ft. These increases have come about in part due to the need to prevent fractures from losing width, but that’s not the only reason. “Proppant per lateral foot is increasing so much because we’re adding more and more clusters, so we have to add more proppant to prop those additional clusters and additional fractures.”
On the other hand, the space between these clusters has been decreasing, Mr Cadotte said. In 2011, cluster spacing typically ranged from 45 to 65 ft. “Over time, operators realized that the risk of over-stimulating the formation was low because the permeability was so low,” he added. Cluster spacing decreased to 30 ft in 2015, then to 20 ft in 2016. Now, cluster spacing of 15 to 20 ft is commonplace in new Haynesville completions. With its ultra-low matrix permeability, Haynesville production can be improved when there are more fracture initiation points along the lateral. Because of the limited drainage radius of fractures in the play, operators see production benefits when cluster spacing is decreased.
However, adding more clusters per stage and decreasing cluster spacing can reduce cluster efficiency. As fractures propagate, the interaction with the formation can stress nearby clusters. Some Haynesville operators responded to this phenomenon by decreasing stage length and completing more stages per well, Mr Cadotte pointed out. He noted that stage length – the distance from plug to plug – decreased from 300 to 400 ft in 2011 to 150 to 200 ft in 2012. During this time period, cluster spacing remained the same. “Wells were being completed with more stages and fewer clusters per stage to promote cluster efficiency,” he said.
Another more recent solution operators have adopted to maximize cluster efficiency is the use of diverters. These agents enabled operators to ensure that multiple, tightly spaced clusters would all be sufficiently treated. At this point, wells completed with a diverter have a typical stage length of 100 to 200 ft, with tight cluster spacing of 15 to 20 ft. “We still having tight cluster spacing, and we’re pumping more clusters per stage,” Mr Cadotte said.
Similar to other US unconventional plays, lateral lengths in the Haynesville have been increasing. Prior to 2015, laterals in the Haynesville did not exceed 7,500 ft. Now, operators frequently drill laterals of up to 10,000 ft with one vertical section, rather than drilling two separate 2-mile long vertical sections to achieve a 10,000-ft effective lateral. The advent of dissolvable plugs has been a major enabler of this trend, Mr Cadotte said. Longer laterals create high downhole friction, which can cause helical buckling or lockup in coiled tubing used to drill out non-dissolvable plugs. Dissolvable plugs eliminate the need to drill out plugs.
To see how these trends have impacted Haynesville production, Halliburton and Covey Park Energy compared a legacy and an optimized well in the Louisiana Haynesville. The two wells are located a half-mile apart. The legacy well, completed in 2010, has a 4,000-ft lateral, 13 stages and 80-ft cluster spacing. It was completed with 1,200 lb/ft of proppant. The optimized well, completed in September 2016, was drilled with a 7,100 ft lateral, has 39 stages and 30-ft cluster spacing. A diverter was utilized in this completion. The operator pumped 3,250 lb/ft of proppant. Overall, proppant per lateral foot increased by 181% from the legacy well to the optimized well, and proppant per cluster increased 32%.
A production comparison between the legacy and optimized well showed that the EUR from the optimized well was higher than that of the legacy well. The legacy well had a steep decline curve when its daily gas rate was plotted against cumulative production. The optimized well’s production rate remained high and flat over the same cumulative production volumes, Mr Cadotte said. “Adding more clusters throughout the lateral and optimizing cluster efficiency either by using a diverter or limiting the number of clusters per stage has greatly increased the number of conductive transverse fractures and the EUR of Haynesville wells,” he said. DC