Forces which act to produce petroleum hydrocarbons from underground accumulations in earth strata include expansion of the hydrocarbon fluids, influx of extraneous fluids such as water or gas, gravity drainage and naturally occurring compaction and consequent porosity reduction of the reservoir rock material. The present invention relates to recovery of petroleum substances by increasing compactive forces through an increase in the effective overburden pressure on the underground accumulation.
Petroleum hydrocarbons are contained within the pores of reservoir rock material, usually under an initial pressure which is related to the subsurface depth of the accumulation. Petroleum containing earth strata most commonly are limestones or sands. Many of the sand materials are friable loose or unconsolidated. These latter, having a granular nature, support the overburden by point contact between the grains. Also, the pressure of the fluids contained in the pores of the sand furnishes additional support for the overlying strata. When this pressure is decreased by production of petroleum substance through wells the point contacts of the grains have to increasingly sustain the weight of the overburden. When the contact pressure between the grains becomes sufficiently intense grain surface at these contacts may give way or fail thus compacting the earth strata and decreasing the porosity thereof. In other words, a part of the overburden weight is transmitted to the fluid to cause an increase in fluid pressure which in turn causes flow toward lower pressure, usually a well bore.
Compaction and porosity decrease may account for a substantial portion of the oil produced. It has been reported (Transactions of the Society of Petroleum Engineers of AIME, volume 261 (1976), page 1113) that almost half of the petroleum recovered from a major oilfield in Venezuela has resulted from compaction. Another result of compaction frequently is an increase in the amount of solid material produced with the petroleum substance.
When earth strata solid material associated with an accumulation of petroleum substance is produced with that petroleum substance it is obvious that the recovery has been 100 percent complete from that particular rock material. However, to the best of my knowledge and belief this fact has not been taken advantage of heretofore in the production of petroleum through wells, as contrasted with mining. On the contrary, strenuous efforts have been expended to prevent production of earth solids including sand control by screens, by consolidation and by gravel packing. The intentional production of such earth solid materials is contemplated herein.
Several factors affect the compaction-subsidence phenomenon. Surface subsidence is greatest where large areas of thick and relatively shallow sand reservoirs are pressure depleted by fluid production and where substantial quantity of solids is produced with the fluids. Because compaction is caused by the weight or mass of material overlying the petroleum being depleted an increase in that mass will cause additional compaction. Seismic waves or vibrations have the effect of increasing overburden mass because they destroy natural bridging of the particles of earth strata above the accumulation and more of the mass is transmitted directly downward. In other words, vibrations hasten the onset and amount of compaction and incidentally increase surface subsidence. Man made vibrations are contemplated herein as an integral part of the present invention.
Petroleum substances usually are recovered from undergound accumulations through wells drilled more or less vertically from the surface of the earth. Horizontal wells such as those disclosed by various patents including those issued to Leo Hanney, notably U.S. Pat. Nos. 1,634,235 and 2,280,851 may conduct fluids to vertical wells to be lifted to the surface. Both vertical and horizontal wells are employed in the present invention.
Mechanical enlargement of a well bore to extract bitumens, tars or other petroleum substances is disclosed in U.S. Pat. No. 3,472,553 issued to B. H. Miller and in U.S. Pat. No. 3,881,775 to McPherson, et al. The present invention may be employed in connection with such mechanically enlarged well bores. Increased compaction will result in more of the bitumen bearing material being brought into contact with the solvent and rotary agitating tool described in the above-mentioned patents.
A hydraulic jet well under-reaming process is disclosed in U.S. Pat. No. 3,155,177 issued to A. B. Fly. Hydraulic underground mining is the subject of U.S. Pat. No. 3,797,590 to Archibald, et al. The present invention is applicable to and will improve such processes. In each instance compaction of the earth strata places more of the desired substance within reach of the path of the hydraulic jet action.
Indeed, if either the mechanical or the hydraulic devices of the above-mentioned patents initially is directed toward the base of the earth strata bearing the desired material, the compactive and vibrative forces of the present invention cause the strata above the devices to collapse and fall into the lower area where the tools and devices are most effective. Also, the angle of repose of collapsed earth strata is decreased by the present invention. Such additional slumping further increases the amount of material which can be reached effectively in such mechanical and hydraulic mining operations.
The use of slip-joint casing and other means to avoid damage to wells in areas of subsidence are well known in petroleum operations and may be used in connection with the present invention. However, it is desirable from a recovery standpoint as well that compaction occur as distant from the axis of the producing vertical well as is practical to achieve and these two factors favor the use of enlarged well bores and horizontal wells connected to vertical wells. It has been noted above that the equilibrium or balance between the mass of an overburden and compaction of underlying earth layers is destroyed by reducing fluid pressure in one or more of the lower strata. Similarly, increasing fluid pressure reverses the balance. Sudden changes in pressure are more effective than gradual changes in compacting and settling earth strata. Included in the present invention is to alternately raise and lower fluid pressure in the accumulation to promote compaction and porosity reduction.