Ground surface subsidence is commonplace over aquifers in the agricultural and commercial water-production industry. It is not unreasonable to expect that subsidence also might be occurring over hydrocarbon-bearing reservoirs actively being produced. The Rubey-Hubbert equation of 1959 expresses an equilibrium relationship between overburden, grain-to-grain stress, and pore pressure (Rubey et al., 1959). As pore pressure continually diminishes with continued production of oil or gas, the increased stress on the mineral structure allows compaction of shales to occur. The result is the expulsion of pore water from the shales as the structure yields to the weight of overburden. If compaction is to take place, then where does the expelled water go? It can be entrained in the produced fluids in the form of an un-predicted water cut. This is a little recognized part of hydrocarbon-recovery history.
Subsidence is a common occurrence above aquifers and some hydrocarbon-bearing formations undergoing production. Where subsidence overlying hydrocarbon-bearing reservoirs has been observed, some water cuts might have been difficult to explain. Production from permeable beds, whether over pressured or at normal pressure, ultimately will result in a decline in pore pressure. This decline allows water to be expelled from shales and clay shales and to search for relief in permeable beds where production takes place. Along with the expulsion of water, compaction occurs and subsidence might be observed. If the water subject to expulsion can find no relief, further compaction will cease. The occurrence of compaction signifies that water has been expelled from the clay shales and has found relief in permeable beds. This relief can be in the form of water cut in produced oil and gas. Along with this pressure decline two things can happen. Water will expand and subsidence might occur. These are not related events, but both result from a decline in water pressure. Each is an independent event, but both occur simultaneously. They are explained below.
First to be cited is water expansion. The bulk modulus of elasticity can be used to estimate the amount of expansion that water will undergo as pressure declines. Compressibility is the reciprocal of bulk modulus. As bulk modulus increases, compressibility decreases, and vice versa. Water is nearly incompressible, but does have a finite bulk modulus, therefore, it does have a finite expansion with pressure decline no matter how relatively insignificant it might be. Subject to permeability constraints, and in compliance with Pascal's Law, water in the shales and permeable producing beds will expand simultaneously with the decrease in pore pressure, slowing the rate of pressure decline and adding to the volume of formation fluids by whatever measure.
Subsidence usually is thought of as a surface event. It is a surface event, but it is far from being the main event that occurs below the surface. Subsidence from millimeters to multiple meters is commonplace where production has reduced pore pressure in the underlying permeable beds.
