In situ gasification may well be limited by the ability to predict and control roof behavior and subsidence. This is particularly true in Texas where lignite seams are overlain by weak rock and often with overlying producing aquifers. A weak overburden implies large closures and significant subsidence. The dried overburden is stronger, and drying due to combustion will tend to reduce closure, but will cause high tensile stresses in the roof so that fracturing is likely. This fracturing will either weaken the roof or lead to collapse, either of which will cause greater subsidence effects, The high reaction temperatures will offset these effects by inducing high compressive stresses, but these will be limited in their time of application, Creep of the lignite and overburden at elevated temperature will cause greater closure but will tend to retain the structural integrity of the roof rock.
With the current resurgence of coal as a major potential energy source new techniques are being investigated for exploiting the abundant deposits below stripping depth in the U.S.A. In situ gasification is one technique which could be both economically and environmentally attractive (1). It depends upon the ability to gasify the coal in place by injection of air, or of a gaseous mixture of steam, nitrogen and oxygen, combustion of the coal in situ and removal of the product gases for use in power generation or the manufacture of chemicals. The behavior of the rocks surrounding and overlying a potential gasification seam will be critical to the process. If no closure of a gasification cavity occurs an excessive void space will exist underground which will lower the heating value of the product gas and cause excessive gas outlet temperatures. (2) On the other hand closure will lead to roof deformation and the development of tensile conditions around the cavity (1) with the possibility of tensile failure and roof collapse. If failure or collapse leads to loss of seal an environmental hazard could occur due to loss of products to the surrounding aquifers and the reaction could be inhibited by excessive water influx. Cavity closure may also lead to excessive subsidence with possible damage to surface structures and disruption of surface drainage patterns. The controlled gasification of coal, therefore, relies upon an ability to predict, or at least assess, the behavior of the cavity roof. The cavity will be subjected to the gravitational, and possibly tectonic, loading common to all underground structures, and the various conclusions on roof behavior which occur in conventional mining will apply. Thus, the roof will deform by an amount dependent upon the deformational properties of the overburden and the depth and size of the cavity. Tensile stresses may occur in the roof leading to the possibility of slip on fracture planes, bedding separation and tensile failure. However an in situ gasification chamber will also be subjected to the high combustion temperatures, leading to the possibility of higher roof deformations and of localized thermal stressing. As an example of some of the problems which may occur the behavior of the roof and overlying rocks in an in situ gasification chamber in Texas lignite is considered. The analysis presented is at this time, idealized and oversimplified.