Efficient and reliable energy supplies are essential for accelerating the growth of every economy. In this context, coalbed methane (CBM) can play a major role in reducing the share of import content and augment the overall energy supply in any country. This article discusses a few techniques that have been developed specifically to address the key challenges: seams that contain most of the gas content and the methods to manage the water produced from it.

Figure 1. Process of gas transportation in CBM reservoirs — fluid production from natural fractures, gas desorption from cleat structures, and molecular diffusion through coal matrix.


CBM is an economic source of pipeline-quality methane that is generated and stored in coal beds. CBM reservoirs are dual-porosity media where the vast quantity of the gas is stored in the low-permeability coal matrix (primary porosity) by adsorption. The flow to production wells, however, occurs through the coals’ natural fracture system (secondary porosity), which stores relatively small amounts of gas because the coal matrix has practically no permeability, as shown in Figure 1. Reservoir pressure must be reduced below the effective partial pressure of methane (critical desorption pressure) to allow two-phase flow of gas and water, allowing methane to flow from its adsorbed state in the coal to gas phase and into the well bore.


CBM scenario in India

India is endowed with rich deposits of coal and lignite in different sedimentary basins of varying dimensions. The bulk of the coal resource of 235 billion tons (241 billion tonnes) is contained in older basins. Large lignite deposits of 98 billion tons (100 billion tonnes) occur in younger basins of Gujarat, Rajasthan (Western India), and Tamil Nadu (Southern India). A characteristic feature of younger basins is the occurance of very thick coal and lignite seams — 65 to 262 ft (20 to 80 m) over a large stretch of the coal/lignite fields. In fact, one of the thickest seams (453 ft or 138 m) is in the Indian coal field of Jharia. Large resources of high-grade coal in the country provide ample opportunities for harnessing this source of non-
conventional gas.

Figure 2. Production performance graph of CBM reservoirs with high rates of water production in the initial stages.

Challenges

The major challenge is that CBM wells produce more water initially when compared to conventional reservoirs, as shown in Figure 2. Removal of this water by pumping is necessary because it helps to lower the pressure in the reservoir, and this process stimulates desorption of methane from the coal. The problem with the disposal of water from the wells is the high cost, ranging from US $400,000 to $1.2 million, depending on depth and stimulation type. The total disposal cost of water to bring it to the surface is approximately $1 to $4 per barrel.

High capital cost is a restriction for small independent operators. Some of the factors attributed to the disposal costs include pipeline maintenance, repair costs, electrical costs to operate pumps, virtually round-the-clock staffing to operate electrical generators, longevity of the facility, depth of the injection well, and chemical treatment to disinfect the water so
that it can be reused for livestock.

If the produced water is disposed as-is into the environment, it creates problems such as ground water depletion, ground water contamination through the presence of methane and degradation of the surface water due to mineral content.


Parameters such as total dissolved solids (TDS), electrical conductivity, and sodium adsorption ratio should be monitored before the disposed water is put to reuse. CBM water does not provide enormous benefits to agricultural land owners, but produced water provides oil and gas operators with flexible, cost-saving water management options for their own use.

Solutions

Figure 3. The figure on the left represents the sample at the start of the experiment, and the sample on the right shows the sample after the experiment has been carried out.

Generally, the following methods have been adopted to dispose of produced water from CBM reservoirs:
• Injection into aquifers depleted by CBM production;
• Discharge of water after treatment as required as surface flows, for example into sewage canals, resulting in improved stream flows with adequate mitigations against negative impact; and
• Use in various operations such as coal mines, dust depression on roads, enhanced oil recovery, and as cooling tower water for various industrial operations.

Proposed method

The present study used a test to examine improvement in the quality of water by direct filtration seed as the coagulant. The goal was to assess the suitability of the method for water treatment in rural areas in developing countries. The technique mainly concentrates on purification systems in cases where these countries lack proper commercial facilities. This focuses on achieving purification using minimum energy resources, which contributes to energy optimization and can meet the environmental norms.

In this test, a one-liter water sample was loaded with solids content of 5,000 parts per million (ppm) of TDS. Two seeds of Strychnos Potatorum were added to the solution, which was left undisturbed for 96 hours. The experiment was conducted at room temperature and atmospheric pressure. After four days, a sample was collected from the jar. The TDS was reduced to below 1,000 ppm. The samples collected are shown in Figure 3.

There are several advantages to this process:
• It does not involve any cost as the seeds cost very little compared to other systems and the loss of resource can be prevented;
• It can be used in rural areas where technology and work force are very difficult to obtain;
• It does not require any energy for purifying the produced water like other conventional purification processes; and
• It can have applicability to oilfield waters. An added plus is that it ensures operational safety because it does not involve high pressure or temperature.
There are also disadvantages:
• The applicability of this process
in rural areas is limited to the availability of the raw materials (seeds) in large enough quantity to carry out the process on a
commercial scale;
• It is a slow process and requires retention time for the coagulants to separate the dissolved solids from water; and
• As it cannot reduce the concentration of the dissolved solids to low ppm values, it is recommended as one of several processes to carry out multistage operations for better purification.

Exploring opportunities

With massive remaining coal reserves and growing interest in CBM opportunities, the Indian coal industry should identify the importance of exploitation of the coal reserves for CBM. Doing so would help the oil and gas operators to gain profits by managing water production through adoption of the above-mentioned solutions. It can also contribute to the energy conservation for the country. Interest has grown in this field because it can be used as a location to collect and store harmful emissions. Coal, the once valuable energy source that became known as a dirty source of energy, is now poised to supply clean energy
and a solution for pollution.