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This paper evaluates dominant and currently applied completion methods by comparing wells in the Middle Bakken continuous resource play that have used all identified systems. The authors discuss these completion designs and methods and fracturing operations that are often executed. Assessment of viability and production efficiency of multiple-entry- point fracturing-sleeve-system (MEFSS) completions as compared with other completion methods is the primary focus of the complete paper.

Introduction

The three most common completion techniques in the Middle Bakken are plug and perforation (P&P), single- entry-point fracturing-sleeve systems (SEFSSs), and MEFSSs. Traditional P&P techniques were developed initially for vertical-well applications and then adapted to horizontal wellbores. As a result, composite-fracturing-plug de- sign had to evolve for horizontal-well deployment and pumpdown capabili- ties. Cementing, typically the isolation method for P&P-completed wellbores, is also difficult in horizontal wellbores. Because of the need for an interven- tionless completion system to facili- tate compartmentalized stimulation de- signs, development and use of SEFSS technologies to overcome many of these challenges came into accepted practice.

SEFSS technologies use specifical- ly sized balls that are dropped from surface to actuate sleeves that are pre- installed with the completion liner string. These actuation balls also pro- vide progressive downstream isolation as the treatment is pumped sequentially from toe to heel. Fracturing-sleeve sys- tems can be used in conjunction with openhole annular isolation packers or cemented in place. These systems do not require millout to ensure that a tar- get zone can produce, although the ball seats are typically milled in the Bakken system. Fracturing-sleeve technologies also help eliminate the use of water and chemicals because the isolation balls do not have to be pumped to depth post- flush as in a P&P technique. Because the actuation balls can be launched and landed on the fly, hundreds of barrels of water are typically saved on each indi- vidual treatment.

With fracturing-sleeve technology, many tools must be deployed to depth as part of the completion liner string. This increases the complexity of run- ning the completion casing after the wellbore has been drilled when com- pared with a conventional liner-running operation. While steps can be taken to overcome these installation challenges, fracturing-sleeve systems are often lim- ited in the number of fracturing stages that can be targeted because of the use of actuation balls and ball seats of grad- uated size against the completion tu- bular geometry. The use of graduated balls and ball seats can also have an effect on the rate at which fracturing stages can be pumped, especially for smaller-sized ball seats toward the toe of the completion.

Background

North Dakota Middle Bakken well- bores are typically drilled to a vertical-depth kickoff point of 7,000 to 10,000 ft, depending on basin geo- graphy; then, a curve is built to approxi- mately 90° in 600 to 1,200 ft, yielding a horizontal-wellbore start at a range of 7,500-ft to greater than 11,000-ft true vertical depth. From there, a 5?- to 6?-in. horizontal-wellbore section is drilled up to 10,000 ft, thus cre- ating a total well depth often greater than 20,000 ft. A 7-in.-casing string is run and cemented in place through the curve section of the wellbore. A 4?-in. completion liner string is run to total measured depth and hung off in the 7-in. main-bore casing string. One of three completion techniques is then used to create access to the formation and en- able the fracturing of 30 or more target zones along the horizontal section of the wellbore.

A sample of four wells was then se- lected that were offset to each other, as well as employing each of the com- mon Middle Bakken completion tech- niques. The offset-well sample had simi- lar characteristics and wellbore designs so that viable comparisons could be made. Production results from the wells were then gathered and recorded from public records over the course of sev- eral months. The production levels were then normalized on the basis of report- ed well parameters and graphed so that analysis could be performed. The goal was to determine for the offset-sample group whether one completion meth- od increased wellbore productivity over the others.

Completion Methods

As mentioned previously, wells in the Bakken system primarily apply three common completion techniques. The techniques include the P&P process, SEFSSs, and MEFSSs. The first two of these systems are detailed in the com- plete paper.

MEFSS. An MEFSS is also ball-actuated; however, a single ball will open mul- tiple fracturing sleeves in the target fracturing stage. The fracturing sleeves and isolation packers are installed with the completion liner string and pro- vide multiple entry points to the for- mation. Once again, an actuation ball is dropped from the surface. The ball will land on the ball seats inside each frac- turing sleeve; however, the actuation ball will pass through multiple sleeves, actuating them, until it will then seat and seal on a fixed ball seat in the last fracturing sleeve in the target stage. In- creased pressure will then open the last target fracturing sleeve. The actuation ball then isolates the target fracturing sleeve from previous fracturing stages and diverts fluid flow out the exit ports in the multiple sleeves in that target stage. Often, these exit ports can also be customized to balance outflow. The target zone is then stimulated, and an- other fracturing ball is dropped to ac- tuate the next set of fracturing sleeves in the MEFSS. The process is repeated until all target zones are stimulated. The actuation balls can then either flow back to surface or degrade in the well- bore fluid.

Offset-Well Production Results

Production results from the offset-well sample were monitored over a period of several months to provide the best possi- ble data set for comparison. The graphs in the complete paper illustrate the pro- duction from each of the sample wells obtained from public data sources. Each of the wells in the sample had a short lat- eral design drilled through the Middle Bakken layer of the formation. The wells in the sample used openhole packers as the annular isolation method between fracturing stages. Fig. 1 illustrates the 3- and 6-month production results from the four offset wells that used the three different completion techniques.

Production levels from the four off- set wellbores were then monitored con- tinually and graphed against each other to enable further analysis of the com- pletion techniques used. While all wells could be monitored for a period of at least 6 months, production results for some of the wells could be obtained for a longer period of time. The results depicted in Fig. 2 feature colors cor- responding to the well from which the production results were obtained.

It is important to note that the pro- duction results were normalized for sev- eral well parameters so that the best comparison could be made and analy- sis could be conducted. On the basis of the reported production levels, it ap- pears that the P&P and SEFSS wells per- formed very similarly in both the near and the long term. However, both of the wells that used the MEFSS completion technique had higher production levels, both initially and long term.