A set of experiments was completed that clearly shows that drilling muds can cause large irreversible damage to fractures and dramatically reduce the productivity of wells producing from a natural fracture network. The use of sized fibers as an additive to drilling muds can significantly reduce both the depth and extent of formation damage. Granular additives (such as CaCO3) are much less effective. The use of acid soluble fibers is, therefore, recommended for reducing formation damage in fractured reservoirs.
Production in many of the world's most prolific reservoirs comes almost entirely from systems of fractures in the producing pay that intersect the wellbore These fractures provide excellent permeability for oil and gas to flow into the wellbore even when the matrix permeability is very small. It has been observed in many instances that significant decreases in productivity are obtained when drilling through these producing intervals with water or oil-based muds. These declines are sometimes noted even in situations where drilling is performed underbalanced It is surprising, therefore, that there is very little work reported in the literature investigating formation damage in such fractured reservoirs.
In this report, the extent and depth of formation damage induced by mud solids as they invade the fracture is discussed. Two types of bridging additives were studied:
long, slender fibers and
granular additives; and the bridging mechanism was investigated.
A great deal of work has been done on lost circulation additives for drilling muds that are used to prevent large scale losses of mud to the formation. Lost circulation problems become particularly acute when drilling through naturally fractured zones and through zones in which fracturing can easily be induced if the mud pressure exceeds the minimum horizontal stress.
Lost circulation materials (LCMs) are usually designed to accomplish two goals:
to bridge across the face of fractures or vugs that already exist and
to prevent the growth of any fractures that may be induced during drilling.
This strategy has led to the development of a whole range of additives that seem to work under some conditions but not in others. There are some general rules of thumb that seem to apply to lost circulation problems:
Oil-based muds are more prone to lost circulation problems than water-based muds.
The size distribution, concentration, shape and method of application of the fluid loss additive is crucial to the success of a treatment.
Past data suggests that muds that form mechanically strong filter cakes across the face of a fracture tend to minimize lost circulation problems. In addition, if induced fracturing is a major mechanism for lost circulation, then lost circulation materials that cause effective tip screen outs to prevent fracture growth may be used.