In conventional oil and gas reservoirs, the primary reason for stimulation is to repair formation damage. But what causes formation damage? And how do we assess it?
Formation damage describes any negative change in the ability of oil and gas to reach or flow into the well. During drilling and completion operations, some formation damage is inevitable. For instance, drilling mud, which is crucial to the lifting of drill cuttings and the cooling of the bit, can cause formation damage by forming a mud cake and by the filtrate (liquid part) leaking off into the formation. In such a case, the drilling mud is a “necessary evil” that solves one or more problems but causes others. The goal of stimulation is to mitigate or repair the effects of formation damage in a cost-effective manner.
Formation damage typically occurs in the region of reservoir rock surrounding the borehole, within a few feet of the well. The low-permeability or damaged region that develops in this area is referred to as the “skin zone”. Any fluids entering or exiting the well need to be able to pass through this region. The amount of formation damage is measured by a term called “skin factor”. If a well is not damaged, the skin factor will be zero, but if a well is damaged, the skin factor will be a positive number such as +3 or +7.
Causes of Formation Damage
Formation damage occurs when the tiny passageways between pore spaces become clogged, usually with small particles or viscous fluids. One way that fine particles are introduced into the well and reservoir is during drilling and cementing of a well. In addition, dissolved materials from casing or tubing, poorly filtered water supplies, brines and other materials can precipitate inside pore spaces, reducing the size of the pore throats. Swelling clays, which react with the fresh water used in drilling, can expand, blocking pore throats and decreasing their size as well.
Smectite clay is one type of swelling clay. When smectite comes in contact with fresh water, it will swell – possibly up to 32 times its original volume – and potentially choke off the pore throats. This is one reason that drilling muds and completion fluids are frequently made of a brine (salt) solution and not fresh water. Salts prevent the clays from swelling.
Crushing and compaction of pore spaces near the well during the perforating of well casings in cased hole completions also reduces the ability of liquids to migrate through the skin region. Even during the production phase of a well, the permeability of the reservoir can be reduced by the formation of scale or the deposition of paraffin or other solid hydrocarbon components.
Detecting Formation Damage
Since formation damage occurs within the solid rock surrounding a borehole thousands of meters underground, it can be quite difficult to diagnose its cause. However, detecting the damage once production has started is possible by carefully observing changes in production through time or comparison of the production behavior to offset wells or other similar formations. Once formation damage has been detected, operators can decide which, if any, remedial operations would be effective and economical. It can be difficult to determine exactly what the cause of formation damage is in a given well, therefore, engineers frequently have to eliminate unlikely causes and narrow down to the most likely suspects.