Combined Application of Screen Hole Protection and CO2 Phase Change Fracturing for Low-Permeability Soft Coal Seams

Literature Overview

This technical topic addresses a specialized application in coalbed methane (CBM) extraction, focusing on the combined use of screen hole protection technology and CO2 phase change fracturing to enhance permeability in low-permeability soft coal seams. While this topic falls outside the traditional cladding and bimetal pressure vessel domain, it intersects with pressure vessel and tubular equipment design, particularly in the design of drilling pipes, screens, and fracturing tools that must withstand harsh downhole conditions.

Technical Background

Low-permeability soft coal seams present unique challenges for CBM extraction. The coal matrix permeability is typically below 1 mD, making conventional hydraulic fracturing ineffective due to poor fracture propagation and closure. Additionally, soft coal is prone to collapse and caving during drilling, which damages the screen holes in the production pipe and reduces well deliverability. The combined technology addresses both challenges: screen hole protection prevents coal ingress during drilling and completion, while CO2 phase change fracturing creates and maintains fractures in the coal seam.

Screen Hole Protection Technology

The screen hole protection technology involves coating or treating the screen holes in the production pipe to prevent coal particles from entering and clogging the screen during drilling and completion. The protection methods include:

1. Chemical coating: applying a resin or polymer coating to the screen holes to seal them during drilling, then dissolving or removing the coating after completion
2. Mechanical plug: inserting temporary plugs into the screen holes during drilling, then retrieving the plugs after completion
3. Cement sheath: cementing the screen pipe with a thin cement sheath that is later perforated to expose the screen holes

The protection material must withstand drilling conditions (high temperature, high pressure, mechanical abrasion) and be removable without damaging the screen. The typical protection material is a thermoset resin with a cure temperature of 100–150°C and a removal temperature of 80–120°C.

CO2 Phase Change Fracturing Technology

CO2 phase change fracturing exploits the phase transition of carbon dioxide from supercritical to gas phase to create fractures in the coal seam. The process involves:

1. Injecting supercritical CO2 (above 31°C and 73.8 bar) into the coal seam through the production pipe
2. As CO2 enters the cooler coal seam, it undergoes phase change from supercritical to gas, expanding in volume by 100–200 times
3. The expansion pressure creates fractures in the coal matrix, enhancing permeability
4. After fracturing, the CO2 is partially recovered and recycled

The key advantages of CO2 fracturing are: (1) lower pumping pressure than water-based fracturing, (2) reduced water usage, (3) ability to fracture low-permeability formations, and (4) environmental benefits of CO2 sequestration.

Combined Application Strategy

The combined application involves a sequential process:

1. Drilling and completion with screen hole protection
2. Removal of screen hole protection material
3. Injection of supercritical CO2 for phase change fracturing
4. Well stimulation and production testing

The screen hole protection ensures that the production pipe remains intact during drilling and completion, while the CO2 fracturing enhances the coal seam permeability for efficient CBM extraction. The two technologies are complementary: without screen protection, the CO2 fracturing would be ineffective due to screen clogging; without CO2 fracturing, the low-permeability coal seam would not produce sufficient gas flow.

Technical Parameters and Equipment Requirements

The production pipe must be designed to withstand the combined effects of CO2 corrosion, H2S corrosion (if present), and mechanical loading from coal compaction. The material selection follows API 934 for casing and tubing, with specific considerations for CO2 service. The screen pipe is typically made of 13Cr martensitic stainless steel or 316L austenitic stainless steel, depending on the corrosion environment.

Engineering Practice and Quality Control

In a field application in the Ordos Basin, the combined technology was applied to a low-permeability soft coal seam with an initial permeability of 0.5 mD. The screen hole protection was applied using a thermoset resin coating, and the CO2 fracturing was performed at 100 bar and 40°C. The post-fracturing permeability increased to 8.5 mD, and the initial gas production rate was 5,000 m³/day. The well continued to produce at 2,000 m³/day after 12 months, demonstrating the effectiveness of the combined technology.

Quality control for the combined application includes:

1. Screen hole protection verification: visual inspection and pressure testing after protection removal
2. CO2 injection monitoring: pressure, temperature, and flow rate recording throughout the injection process
3. Post-fracturing testing: flow testing, pressure testing, and permeability measurement
4. Long-term monitoring: production rate, gas composition, and well integrity monitoring

Study Insights and Reflections

The combined application of screen hole protection and CO2 phase change fracturing represents an innovative approach to CBM extraction in challenging geological conditions. The technology addresses two critical challenges simultaneously: wellbore integrity during drilling and completion, and reservoir stimulation for gas production. The engineering challenge lies in integrating two distinct technologies into a seamless operational sequence, with proper material selection, process control, and quality assurance.

From a pressure vessel and tubular equipment perspective, the technology highlights the importance of material selection for harsh downhole environments. CO2 and H2S corrosion, mechanical loading, and thermal cycling all impose demanding requirements on the production pipe. The selection of 13Cr or 316L stainless steel reflects the need for corrosion resistance and mechanical strength, with the specific choice depending on the severity of the corrosion environment.

The broader implication is that advanced CBM extraction technologies require interdisciplinary engineering: geology, drilling engineering, materials science, and pressure vessel design must work together to develop effective and reliable extraction systems. The engineer must understand the interplay between geological conditions, equipment design, and operational parameters to optimize the extraction process and ensure long-term well performance.