Summary Hydraulic fracturing in coals has been studied extensively over the last two decades; however, there are factors that were often ignored or incorrectly diagnosed, resulting in screenouts. Assuming that a majority of the perforations are open and there are no problems with the stimulation fluids, screenouts during coal hydraulic-fracture treatments can be attributed to either high pressure-dependent leakoff (PDL), high process-zone stress (PZS) or in some cases both. The objective of this work is to discuss, help identify, and present solutions to address these reservoir-related issues such that screenouts can be avoided in optimized refracture treatments and new well stimulations. The tools for identifying these reservoir-related parameters include a diagnostic fracture-injection test (DFIT) and a grid-oriented fully functional 3D fracture simulator with shear decoupling. An example for each respective case is presented in this paper. In the first example, in which high PZS was considered to be the dominant reason for screenout or pressure out, the well was restimulated successfully by implementing the solutions presented in this paper. In the second example, in which high PDL was considered to be the main reason for screenout, there were several wells in the same project area that exhibited the same behavior resulting in screenouts. After implementing the solutions presented in this paper to address high PDL, all new wells were stimulated successfully without any issues.

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 (a) minimize damage in coals, and (b) 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: (a) minimizing damage caused by gel, (b) maximizing regained permeability, (c) containing height growth, and d) 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.