1. Field of the Invention
Embodiments disclosed herein relate generally to chemically degradable gels and breaker systems for degrading gels formed downhole.
2. Background Art
Lost circulation is a recurring drilling problem, characterized by loss of drilling mud into downhole formations that are fractured, highly permeable, porous, cavernous, or vugular. These earth formations can include shale, sands, gravel, shell beds, reef deposits, limestone, dolomite, and chalk, among others. Other problems encountered while drilling and producing oil and gas include stuck pipe, hole collapse, loss of well control, and loss of or decreased production.
Induced mud losses may also occur when the mud weight, required for well control and to maintain a stable wellbore, exceeds the fracture resistance of the formations. A particularly challenging situation arises in depleted reservoirs, in which the drop in pore pressure weakens hydrocarbon-bearing rocks, but neighboring or inter-bedded low permeability rocks, such as shales, maintain their pore pressure. This can make the drilling of certain depleted zones impossible because the mud weight required to support the shale exceeds the fracture resistance of the sands and silts.
Other situations arise in which isolation of certain zones within a formation may be beneficial. For example, one method to increase the production of a well is to perforate the well in a number of different locations, either in the same hydrocarbon bearing zone or in different hydrocarbon bearing zones, and thereby increase the flow of hydrocarbons into the well. The problem associated with producing from a well in this manner relates to the control of the flow of fluids from the well and to the management of the reservoir. For example, in a well producing from a number of separate zones (or from laterals in a multilateral well) in which one zone has a higher pressure than another zone, the higher pressure zone may disembogue into the lower pressure zone rather than to the surface. Similarly, in a horizontal well that extends through a single zone, perforations near the “heel” of the well, i.e., nearer the surface, may begin to produce water before those perforations near the “toe” of the well. The production of water near the heel reduces the overall production from the well.
During the drilling process muds are circulated downhole to remove rock as well as deliver agents to combat the variety of issues described above. Mud compositions may be water or oil-based (including mineral oil, biological, diesel, or synthetic oils) and may comprise weighting agents, surfactants, proppants, and gels. In attempting to cure these and other problems, crosslinkable or absorbing polymers, loss control material (LCM) pills, and cement squeezes have been employed. Gels, in particular, have found utility in preventing mud loss, stabilizing and strengthening the wellbore, and zone isolation and water shutoff treatments.
In attempting to cure these and other problems, crosslinkable or absorbing polymers, loss control material (LCM) pills, and cement squeezes have been employed, each of which may include materials such as lignins and lignosulfonates, as well as lignin and lignosulfonate gels.
In many wells, water-based and oil-based muds are both used. Water-based muds are generally used early in the drilling process. Later, oil-based muds are substituted as the well gets deeper and reaches the limit of the water-based muds due to limitations such as lubricity and well bore stabilization. The majority of gels employ water compatible gelling and crosslinking agents. There is, however, a dearth of methods using non-aqueous gels which are compatible with oil-based muds.
Further, while there have been an increasing number of developments in gel technology for use downhole, stable gels that may subsequently be chemically degraded by application of a breaker system may find use in situations where a gel has been misplaced or needs subsequent removal from the formation.
Accordingly, there exists a continuing need for developments in gel technology and means for chemically degrading gels formed downhole.