Gas Content Without Coring
Sigra is able to determine the gas content of coals and other strata, such as shales, using a system to collect all gas released during overbalanced drilling with mud. This process involves using a (rotary) seal at the well head allowing all fluids produced from the well to be captured and directed to a separator, usually in the form of a hydrocyclone.
The gas released from the cuttings is collected from the separator and its volume and composition measured. The mud and cuttings pass from the cyclone. In high-flow operations, the mud and cuttings will move on to a shale shaker from which the cuttings can be sampled and tested using the same process as that described for chip desorption.
Figure 1 shows schematically the setup for this process. This is ideal for use on a production coal seam gas operation. It is also highly suitable for use on shales where core is expensive to obtain and disruptive to the drilling operation.
Following drilling, the hole should be geophysically logged to determine the volume and nature of the material drilled. By careful monitoring of drilling rate, depth and the mud flow rate Sigra can determine the origin of the cuttings. The method is inherently more accurate than core desorption because there is no reliance on lost gas determination – all gas released is captured. The use of the technique is reported by Gray, Singh and O’Brien (2013).
The typical applications for this are:
- Initial exploration drilling – where the client wishes to avoid coring.
- Production hole drilling – where the client wishes to firm up knowledge of gas contents during the drilling of production holes.
- Where free gas is expected in such formations as porous sandstones where the gas would be lost on coring.
- Where the entire gas content of the stratigraphic sequence needs to be measured.
The system is not intended to replace gas content determination from core where this is targeting one or two well known coal seams or where core is required for other purposes. The system is however particularly useful in that it provides an absolute measurement of gas content through all the strata drilled.
The system can be installed on virtually any open hole rig that has a conventional mud handling system comprising mud tanks and a shaker. The additional equipment that is required comprises:
- Drilling rod head position measurement device.
- A dribble prevention valve at the water swivel.
- A pressure relief valve behind the bit.
- A rotary seal at the top of the hole. This may be fitted above the casing or over the BOP.
- A rate counter on the mud pump.
- A cyclonic separator placed between the outlet of the rotary seal with outlets for the gas flow meters and to the shaker.
- Gas flow and composition logging equipment.
- The ability to sample cuttings off the shaker.
These are simple additions to most drilling operations.