Approximately 80 percent of the world's sandstones can be classified as "dirty". However, dirty sandstones contain only 15 percent of the world's oilfields. Most oilfields have been discovered in clean sandstones. Differences in success ratios between clean and dirty sandstones suggest either that:
dirty sandstones do not contain recoverable hydrocarbons (particularly oil), or
hydrocarbons are more difficult to extract from dirty sandstones than clean sandstones.
Commercial quantities of hydrocarbons have been discovered in dirty sandstones.
This strongly suggests that their hydrocarbon potential is good.
However, detailed studies of the composition of dirty sandstones indicate that they are highly susceptible to formation damage during both drilling and stimulation. Care and attention to rock mineralogy and fluid composition can minimize or prevent formation damage in dirty sandstone reservoirs. With properly designed stimulation procedures, 80 percent of the world's sandstones (the dirty sandstones) become highly prospective and good exploration targets.
The terms "clean" and'dirty" sandstones are used (and often misused) widely in the petroleum industry. Clean sandstones are comprised dominantly of sand grains which are chemically stable and physically durable. On the other hand, dirty sandstones are comprised dominantly of sand grains which are chemically unstable and physically non-durable (Figs. 1 and 2). Note that the terms clean and dirty do not involve the amount of clay or shale in the sands. Both clean and dirty sands can be clay-rich or shaly. Distinction between clean and dirty sands is thus based on the composition of the sand sized particles, not the relative abundance of clay or shale.
POROSITY AND PERMEABILITY REDUCTION IN DIRTY SANDSTONES POROSITY AND PERMEABILITY REDUCTION IN DIRTY SANDSTONES Reservoir stimulation in dirty sandstones has, as its principal aim, the enhancement of rock permeability. In order to predict the effects of specific permeability. In order to predict the effects of specific stimulation efforts, it is necessary to first understand the reasons for low permeabilities and porosities in many dirty sandstones. Specific stimulation efforts should be designed to attack either the principal cause of porosity-permeability reduction, or those causes which can be treated with the present state of the art.
Two of the most important methods of porosity reduction in dirty sandstones are:
Ductile grain deformation during burial compaction, and
Addition of natural cements to the pore systems.
The actual choice of a stimulation technique should depend upon which of these two processes is responsible for most porosity reduction in a specific reservoir. Furthermore, the actual choice of drilling and stimulation fluids should depend upon the composition of both the sand grains which comprise the framework of the rock, and the natural cements in the pore systems. In reality, compaction and cementation pore systems. In reality, compaction and cementation actually compete in a race to destroy original porosity. The winner of this race often determines porosity. The winner of this race often determines the degree of success of individual stimulation procedures. procedures.
Compaction of sediments during burial can result in significant porosity occlusion through deformation of ductile grains. Dirty sandstones contain high proportions of ductile grains (grains which alter proportions of ductile grains (grains which alter their shape significantly under compaction). Ductile grains flow into and conform with available pore space with increasing depth of burial (Figs. 3 and 4). The presence of 30 to 40 percent ductile fragments within a sandstone may result in total porosity occlusion by ductile grain deformation porosity occlusion by ductile grain deformation during burial compaction. Dirty sandstones are commonly characterized by abundant ductile fragments. Thus porosity reduction through ductile grain deformation is inevitable in most dirty sandstones, particularly if the sediments have been buried to particularly if the sediments have been buried to depths greater than 5000 feet.