Limited Entry Completions
For decades the generally accepted method for horizontal multistage fracturing in tight rock formations has been the Plug-and-Perf type completion (PnP). This is a Limited Entry system which utilizes a series of spaced perforation clusters along each section of the lateral wellbore; referred to as a stage. All clusters within each stage are stimulated simultaneously during a single pumping treatment.
The basic assumption has been that all clusters receive equal stimulation regardless of formation characteristics. Inconsistencies in production results and post stimulation fracture analysis have led to the conclusion that formation factors play a large part in the overall efficiency of PnP completions.
Formation heterogeneity can result in natural stress fractures and variable in situ fracture gradients along the lateral section. This can cause stimulation treatments to concentrate at perforation clusters near natural fractures or at clusters near the lowest formation fracture gradients rather than achieving equal distribution. Consequently some clusters can be overstimulated while others may be pinched off and receive little or no treatment within a single stage.
Single-point Entry Completions
Recognizing this phenomenon over more recent years, various alternative completion technologies have been introduced which accommodate Single-point Entry fracturing. This type of completion commonly utilizes a coiled tubing activated sleeve or a ball drop system. The sleeve is integral to the casing string or liner and is prepositioned at the desired fracture location within each stage to be pumped. The sleeve is opened via mechanical and/or hydraulic means prior to pumping and the fracture position is controlled since treatment enters the formation only through ports in the sleeve.
This prevents variable formation characteristics from affecting the desired fracture placement. In many cases less horsepower has been required to pump the treatment, better real time control has been maintained during pumping operations, treatment modeling was more accurate and improved production results were often realized. Some operators have implemented Single-point Entry type completions on a trial basis but recognized that mechanical limitations could prevent adopting them as a standard for all applications.
Potential drawbacks include the risk of not getting to target depth with coiled tubing in extended reach laterals due to metal-to- metal friction generated with the casing wall. Ball drop systems typically have a finite number of graduated seats which could limit the number of stages available within a wellbore and most ball drop systems leave seats in the hole which have inside diameters smaller than full wellbore. Regardless of trial performance, these limitations have forced some operators to return to the PnP completion which the industry is so familiar with, creating a need for disruptive technologies to address these concerns.
The Focus Turns to Completions
With prolonged low oil prices, operators have placed a high priority on completion efficiencies as this directly impacts estimated ultimate recovery (EUR) and return on investment (ROI) during a well’s life cycle. Accordingly, Single-point Entry versus Limited Entry fracturing has become a key discussion topic. Due to confidentiality, significant completion data and detailed production information is not always readily available to the public in formats useful for comparative completion analysis. However, case history documentation is becoming more available through technical papers, media articles and press releases.
Fracture Efficiency Analysis
Various methods have been utilized to analyze post stimulation results in low perm horizontal wells. Temperature and acoustic gages are typical and in a few cases fiber optic gages have been installed with the casing string. Microseismic and radioactive tracer analysis have also been performed. One study of numerous horizontal well production logs (Miller 2011) concluded that almost 30% of perforation clusters do not contribute to production in PnP wells. Another study (Castro 2013) used radioactive tracer data to illustrate that all clusters are not being stimulated with PnP completions.
In a coauthored technical paper one operator in the Bone Springs area of West Texas estimated that as much as 40% of the reservoir was not being stimulated using PnP due to variable fracture gradients. A microseismic survey in the field identified fractures extending far beyond the model calculations and communicating with an existing well bore more than 3,000 feet away. Newfield Exploration Company has coauthored a technical paper documenting the use of radioactive tracers to conclude that approximately 30% of clusters in a Granite Wash PnP gas well were not stimulated. However, the same analysis revealed that all stages in an adjacent Single-point Entry well were placed properly and fully stimulated.
A Sample of Production Comparisons
* In a June 2014 public presentation Whiting Petroleum Corporation cited an example where they completed one well at Missouri Breaks with a cased hole Single-Point Entry system. Initial production was 40% higher than two offset wells with similar laterals completed with the PnP method and 70% higher than one offset well completed in open hole with a sliding sleeve.
* Arrington Oil & Gas Operating LLC coauthored a technical paper in 2014 which documented a Single-point Entry completion in the Bone Springs reservoir in West Texas. The casing was stuck off bottom so the well was completed with a 2,600’ lateral rather than the intended 4,800’ length. The average daily production exceeded three PnP wells on the same lease by more than 100% after 8 months even though those wells had 4,800’ laterals. The production of the Single-point Entry well also exceeded production on four PnP wells with 4,800’ laterals on adjacent acreage by 50% to 100% after 4 months.
* Newfield Exploration Company coauthored a technical paper in 2013 in which two identical wells were drilled consecutively in the Granite Wash gas play in the Texas Panhandle for the purpose of providing controlled comparison data between a PnP completion and a Single-point Entry completion. The Single-point Entry gas well exhibited a production rate approximately 100% higher than the PnP well after 75 days online. An estimated ultimate recovery (EUR) model was simulated in which the Single-point Entry well was estimated to exceed the PnP well by 107% (6,493 MMCFE vs 3,143 MMCFE).
Forward Thinking Industry Leaders Recognize the Advantages of Single-point Entry Technologies
Single-point Entry completion technology offers clear technical advantages in regard to controlled fracture placement and stimulation efficiency. The trending emphasis on choosing the best completion practice for each well application should lead the industry as a whole to give serious consideration to these systems, and Stage Completions strives to remain an innovator and pioneer in this arena as the industry continues to adopt these technologies.
About this Blog
In this blog, we hope to engage the oil and gas community with information about multistage fracturing. We think analyzing and offering our expertise about the ways our industry is evolving will help others seek the most innovative technologies and practices as they become available. If there’s a subject you’d like to know more about, please let us know. Thank you for joining us in the conversation.