What are the best ways to optimize drilling performance in coalbed methane wells?

1 Choosing the right drilling method

There are different drilling methods that can be used for CBM wells, depending on the depth, thickness, and permeability of the coal seams. The most common methods are vertical, horizontal, and multilateral drilling. Vertical drilling is the simplest and cheapest option, but it may not access enough coal seam area to produce enough gas. Horizontal drilling can increase the contact area with the coal seam, but it is more complex and expensive than vertical drilling. Multilateral drilling can create multiple branches from a single wellbore, which can enhance the drainage and production of CBM, but it also requires advanced technology and skills.

2 Using appropriate drilling fluids

Drilling fluids are used to lubricate, cool, and stabilize the wellbore during drilling. However, they can also damage the coal seam and reduce its permeability, which can affect the gas production. Therefore, it is important to use appropriate drilling fluids that have low viscosity, low density, low solid content, and low damage potential. Some of the drilling fluids that are suitable for CBM wells are air, foam, nitrogen, mist, and water-based fluids. The choice of drilling fluid depends on the reservoir pressure, temperature, and gas content.

3 Controlling the wellbore stability

Wellbore stability is a major challenge in CBM drilling, as coal seams are prone to swelling, caving, and fracturing due to stress changes and fluid invasion. These problems can cause wellbore collapse, stuck pipe, and lost circulation, which can increase the drilling time and cost. To control the wellbore stability, it is essential to monitor and adjust the drilling parameters, such as weight on bit, rotary speed, flow rate, and mud weight. Additionally, it is advisable to use casing or liners to isolate the coal seam from the surrounding formations and prevent fluid loss.

4 Applying hydraulic fracturing

Hydraulic fracturing is a technique that creates cracks in the coal seam by injecting high-pressure fluid, which enhances the permeability and gas flow. Hydraulic fracturing is often necessary for CBM wells, as the natural fractures in the coal seam may not be enough to provide sufficient drainage and production. Hydraulic fracturing can be performed before or after drilling, depending on the reservoir characteristics and the drilling method. Hydraulic fracturing can improve the drilling performance by increasing the gas recovery, reducing the water production, and extending the well life.