Stimulation Optimization of Coalbed Methane Wells using Coiled Tubing
Authors: Sharma Ajay 
Published: November 2013

Pinpoint technology pioneers a specific technique wherein target zones are selectively perforated by pumping abrasive fluids through a specifically designed coiled-tubing (CT)-deployed bottomhole assembly (BHA) to perform abrasive jetted perforating. This provides an economical and efficient method of perforating, which is immediately followed by fracture stimulating the interval without removing the CT. This is achieved by pumping the fracturing fluid down the casing-CT annulus. The fractured zone is then isolated after the fracturing stage is complete by means of a sand plug, immediately followed by individually treating multiple additional zones in a similar fashion. This is all performed without having to pull the CT completely out of the well. With sand plugs being the primary mode of zonal isolation, an attempt should be made to achieve a proppant packoff using higher concentrations of sand in the final stages of each fracturing treatment (except the top one).

This pinpoint stimulation method involves the synergistic integration of CT and fracture stimulation disciplines of well services. This paper highlights the lessons learned from both operational and engineering standpoints of CT applications during the process of executing a coalbed methane (CBM) fracturing campaign in the Raniganj field of West Bengal, India. The discussions within this paper focus on engineering best practices, optimization of resource utilization, increment of the number of treated zones per day, and reduction of non-productive time (NPT).

This treatment technique provides an easy and economical method for achieving depth correlation. The small seam thickness of the CBM zones to be fractured requires a precise depth correlation technique to help ensure that the perforations are placed within the target seam. A mechanical casing collar locator (MCCL) tool is employed during this process, which helps correlate the CT depth to the wireline depth. This is why the success of this tool, to a major extent, depends on the accuracy of the wireline logs supplied by the well operator, to which the collar depths are correlated. This paper also discusses the optimization of this tool, which resulted in an increase to the tool life and reduction of downtime caused by unnecessary roundtrips of CT, consequently reducing total fatigue on the CT string.

A prominent feature of this treatment method is the hydrajetting technique, which uses abrasive sand jetting to cut perforations through the casing, through the cement sheath, and farther into the formation. It provides a cleaner and more economical method of perforating compared to conventional perforating techniques. This can result in reduction of entry friction and larger perforation diameters, leading to enhanced communication to the reservoir and lowering of fracture initiation pressure. While perforating the CBM formation, several parameters were manipulated to augment the technical and operational efficiency of the entire process, which are discussed in this paper. Once the perforations are cut, acid is displaced down the CT to clean the cement remains and debris from the perforating. An inherent advantage of using CT in this process is acid being spotted directly into the perforations, causing a more effective cleaning of the perforation set, often characterized by reduction to treating pressure during fracturing stages.

This paper discusses a case in which, during the execution of hydra jet perforating in deviated wells, one of the major challenges encountered was the inability to lift cutting sand from casing (post perforating) using forward circulation (pumped through CT and returns taken through the annulus). Ideally, during a hydrajetting process, cutting sand is circulated out of the wellbore once the cut is complete. However, in these highly deviated sections, because of the size of CT and casing used, there was insufficient annular velocity to lift cutting sand from the well. This issue was resolved using unconventional methods, which are also discussed in this paper.