Formation Damage caused during drilling can have a significant impact on the near wellbore and the ability for a well to deliver to design. The reduction in near wellbore permeability is generally interpreted as a negative and the damage is normally minimised through careful selection of fluids, drilling methods and parameters. The focus on formation damage usually comes during development well planning, where optimum productivity or injectivity are required. Formation damage can also have an impact on core quality when cores are being cut and recovered to surface.
Good quality core is valued by Formation Evaluation and indeed, formation damage specialists, as it enables direct measurement of rock and fluid properties on real samples of real reservoir. It is well known that core retrieved too quickly from reservoir to surface can be physically damaged by “gas expansion”. Previous work has demonstrated that gas expansion damage should only be a major issue in low permeability reservoirs (Zubizarreta, 2013; Hettema, 2002). However, higher permeability reservoir cores can also be damaged (Worthington, 1987; Holt, 1994).
Consider the core surface and a few milimeters in to the core – mud cake and damage occur. The permeability of damaged zone is significantly lower than reservoir rock. In gas reservoirs or when bubble point is reached in high GOR, core retrieval leads to pressure difference across the damaged zone and desaturation of the mud cake damaged zone. If this pressure difference exceeds strength of rock then a thin layer of damaged rock material may develop. This fracturing of the core can render it useless for many of the normal laboratory tests conducted on core.
This paper will present evidence of structural damage developed due to formation damage around reservoir core. We will also present the modeling method used to predict this damage and give some examples of core tripping out schedules designed to protect the valuable core resource. The impact of formation damage on core structural integrity will be explored in detail for the first time and the method presented can be used to predict and avoid structural damage in future coring operations.