This paper examines the effectiveness of a completion technique where ball-activated sliding sleeves (BASS) are cemented into the wellbore. This technique allows operators to change zones between fracture-stimulation treatments by dropping balls from surface, instead of using the conventional pumpdown plug and perforating method. A five-well group completed with this method is compared to its direct offsets that were completed conventionally to draw conclusions. Initial studies show that this completion method reduces the time and cost to complete a horizontal well while yielding production that is equivalent to or better than the offset wells.
Preliminary research shows that the BASS group average 30-day Initial Production (IP) was approximately 25% better than the offset well average, and that the 9-month IP was approximately 33% better than the offset well average. Using the ball-drop completion technique, production increases were accompanied by faster completion times, a reduction in required hydraulic horsepower (HHP), and substantial cost savings. On average, wells that were completed using the cemented BASS showed no cost increase during the drilling phase, and reduced the mechanical completion cost by $20,000 to $40,000 per well when compared to offset wells. The primary efficiency driver during completion is the ability to prepare the well for fracture stimulation and complete multiple stages of fracture stimulation without having to utilize a workover unit. Additional time savings occurs between fracture-stimulation treatments; this method reduces downtime between stages from 3 hours to 20 minutes. This time savings can lead to more efficient use of equipment, which benefits the operator and the pumping company and improves community relations.
The Barnett shale is Mississippian-age marine shale that has proven to be productive when fracture stimulated. While most wells in the early development/exploration stage of the Barnett shale were vertical, the large majority of current wells are being drilled and completed horizontally. A common completion would be a production string of 5.5-in., 17-lb/ft, N-grade casing cemented through the horizontal section to provide zonal isolation and bring the top of cement up to cover the next productive interval. Once the casing is cemented in place, the drilling rig is demobilized and a workover rig is then mobilized to prepare the well for the first stage of fracture stimulation. The cement integrity is then checked by running a cement bond log. A workover rig can prepare the well in two days, with the first day being a bit and scraper run and the second day spent running the tubing-conveyed perforating equipment to perforate the holes to stimulate Stage 1. Once the first zone is perforated, a pump-in test can be performed.
When designing the completion, operators vary stage length from 150 to 550 ft, depending on the local geology, with shorter stages being used in areas where the Viola has pinched out, and longer stages being used in the areas where the Viola provides a suitable barrier from stimulating the water-bearing Ellenberger sands below the Barnett. The number of perforation clusters per stage varies greatly by operator, with some using a single-perforation cluster per stage, while others space six or more clusters as close as fifty feet apart. Many operators manipulate the perforation spacing and stage size in an attempt to control the height growth of the stimulation treatment, avoid stimulating potential geohazards, and target favorable rock.