Abstract

Some of the significant strides made in coal stimulation during the last 25 years can be attributed to the development of new-generation fluid systems (i.e., low gel loading fluids with efficient low-temperature breaker systems that cause less polymer damage in coals). All these new developments were implemented to

  • minimize damage in coals, and

  • maximize production.

Water fracture treatments in coals completely eliminated polymer damage but did not always maximize production. The use of new-generation crosslinked fluids did provide better half-lengths and conductivities but still left residual damage in coals. Based on production, it was confirmed that the benefits obtained with this fluid system outweighed the damage created. To further reduce the damage in coals and obtain better regained fracture permeability, implementing hybrid fracture treatments in coals was considered in this San Juan basin project. The term "hybrid" in this case refers to a water pad followed by crosslinked fluid sand stages. Potential benefits of this technique in coals include:

  • minimizing damage caused by gel,

  • maximizing regained permeability,

  • containing height growth, and

  • lowering cost.

There are two parts to this work. The first part presented in this paper contains the design, implementation, and encouraging initial results obtained from the seven wells in this project, which is in an underpressured area. This is the first project in which hybrid-type treatments have been applied in a low-pressured formation. This paper will discuss the lessons learned from such an application in coals. When sufficient production data becomes available, the second part of this work will quantify the results via reservoir simulation. The second part will also include quantified results from another current hybrid fracture-stimulation project where the coals are slightly overpressured.

Introduction

Historically water fracture and hybrid fracture treatments have been applied to low-permeability reservoirs in the United States.[1] Since its inception, water fracture treatments have also been applied to coalbed methane (CBM) wells.[2,3,4] Water fracture treatments are used in coals mainly because, unlike crosslinked fluid systems, they do not cause damage to coals; they are also less expensive. However, an industry survey of coalbed methane completions in the six CBM basins in the U.S. has shown that crosslinked fluid fracture treatments have outperformed water fracture treatments.[5] The authors attribute this to the proppant carrying capacity of the new-generation low gel loading fluids with efficient breaker systems. The benefits of obtaining better effective half-lengths with these new fluid systems have outweighed the residual damage created by gel. To further improve regained permeability by reducing residual gel damage and still maintaining proppant carrying capacity, hybrid fracture treatments were considered in this project. According to Rushing et al.,[6] hybrid fracture treatments provide the benefits of both gel and water fracture treatments.

A logical requirement when considering a water fracture treatment is whether the formation will have sufficient energy to unload additional water. Although a hybrid fracture treatment uses less water than a conventional water fracture treatment, it still uses more water than a conventional gel fluid fracture treatment. Although pressure transient tests revealed that the coals in the project area were underpressured, hybrid fracture treatments were still considered for the project because the wells operate on rod pumps. It was assumed that these pumps would be sufficient to negate the low reservoir pressure in the coal and unload the additional water that would be injected during the stimulation treatment. The additional water would come mainly from an increase in the pad volume; this incremental volume was not expected to affect the relative permeability to gas significantly.

The fact that the thin, low-viscosity pad fluid used in hybrid fracture treatments creates more confined fractures was one of the main reason that this type of treatment was considered for this project. The design and implementation of this hybrid fracture treatment for the case wells and the encouraging initial results are discussed in this paper. An evaluation of these fracturing treatments will be made when sufficient data from reservoir simulation becomes available; the analysis will be presented in the second part of this work.

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