EFFECTS OF FORMATION DAMAGE ON OIL WELL PRODUCTIVITY
Formation damage has been a constant headache to the oil producing industries as it is considered an impairment of the permeability of petroleum bearing formation with an expensive remediation procedure. Although, the prevention of formation damage is impracticable since every single operation embarked upon in petroleum production is a potential source of damage, it could be controlled. In this project, a well was studied and BHP survey was used from BHP analysis in addition to the information of the well history and reservoir data available. The well was observed to have been damaged with a skin of 115 and a damage ratio indicating the well should have been flowing about two times its present production rate. There are two major stimulation procedures which are the hydraulic fracturing and the matrix acidization in which the latter was used in the case of the damaged well. Well 57XX had a production rate which was initially 1550bbl/day at its peak before undergoing a decline, increased to 2100bbl/day and then continued to flow at an average of 2000bbl/day before a sharp decline and subsequent gradual declination of production rate showing the effect formation damage had on the well 57XX. This in conclusion proved that the matrix acidization technique used to stimulate the well was effective as it led to an increase in the well permeability and hence, increased the oil production rate.
1.1. General Background
Formation damage is generally considered as the impairment of the unseen by the inevitable, causing an unknown reduction in the unquantifiable (Petrowiki, 2015). Also, it is a condition which occurs when barriers to flow develop in the near-wellbore region to give rise to a lower than expected production rate from or injection rate into a hydrocarbon bearing reservoir rock and it requires interdisciplinary knowledge and expertise (Amaefule et al, 1988). It can also be referred to as an impairment to reservoir (reduced production) permeability caused by wellbore fluids used during drilling, completion and work over operations (Petrowiki, 2015).
Oil well productivity on the other hand, is generally considered as the ability of a reservoir to produce hydrocarbons after the well has been drilled and made ready for production. The production stage of oil is the most important stage of a well’s life because it determines if the aim of drilling such well has been achieved or not, and this can be measured by the quantity of crude oil derived or quantity of crude oil which is producible. Formation damage is one of the major causes of decrease in oil production as a result of damage to the formation by reducing its porosity and permeability which also leads to flow restrictions. Flow restrictions into the wellbore create additional pressure drops known as ‘skin’ and reduce well productivity.
Formation damage is known to occur during any stage of a well’s life; from initial exploration, through appraisal, through production and through secondary or tertiary recovery and all these have their various roles which they play in the reduction of oil well productivity. Formation damage indicators include, among others, permeability impairment, skin damage and decrease of well performance. Formation damage according to Porter (1989) is considered not necessarily reversible and what gets into the porous media does not necessarily come out. It is, therefore, better to avoid the occurrence of formation damage rather than trying to restore it. Models for formation damages which have been proven to be verified can be used to avoid or minimize it (Faruk, 2011). Carefully planned laboratory and field tests can also help in providing scientific guidance as well as develop strategies for minimizing the damage. It will, therefore, cause considerable cost for remediation and deferred production. Accurately designed experimental and analytical techniques with the modelling and simulation approaches can be used to understand the evaluation, prevention, remediation and the control of formation damage that leads to low oil productivity.
Formation damage can occur as a result of fluid/rock incompatibility; particle migration and deposition may occur as a function of the chemistry of the clay minerals and the chemical and electrochemical nature of both the natural formation fluid and the drilling fluid. Changes in the pore fluid can also induce clay swelling which in turn reduces the pore spaces in the reservoir and this is considered a form of damage to the formation as it reduces the productivity of the formation.
The occurrence of the fluid/rock incompatibility is not as a result of only swelling of the clay and particle migration and deposition. Formation damage can also occur as a result of the fluid/fluid incompatibility. The incompatibility of the introduced fluid (drilling fluid) and the reservoir pore-fluid which creates emulsion blocks can only be controlled by stimulation techniques that include pre-flush or after flush techniques. Formation damage caused by various fluids introduced into the well is remediated by careful treatment design and quality control. The departure from radial flow in a homogenous and isotropic medium can also be a cause of formation damage. A positive skin may arise from a reduction of the area available to flow and/or a departure from purely radial flow (Harper and Buller, 1986).
Formation damage also has other causes such as the mechanical deformation around a borehole or perforation tunnel, reduction of fluid pressure during production, etc. Thorough understanding of the formation damage mechanism’s stringent measures for its control and prevention, and effective and efficient treatments are the keys for optimum production strategies for oil and gas fields.
The consequences of formation damage are the reduction of the oil and gas productivity of reservoirs and noneconomic operation. Hence, once formation damage has occurred, it is necessary that proper assessment, planning and treatment will require the cooperative efforts and knowledge of the geologists, reservoir engineer and production engineer both in the field and in the laboratory. This combined effort and approach will therefore help to develop effective solutions to the damage. A wide knowledge of the mechanism of formation damage is necessary in order for the engineers and geologists to develop effective, preventive and mitigating procedures.
With recent improvements in technology, laboratory, geology and engineering, it is easier to achieve accurate measurements which can provide the necessary insights into the mechanism, prevention and effective treatment of formation damage (Amaefule et al., 1988). Confidence in formation damage prediction using models cannot be achieved without undergoing field testing as they are necessary for the verification of the models. After the verification of the model, it can then be applied for accurate simulation of the reservoir formation damage and designing effective measures for formation damage (Faruk, 2011).
Formation has varying characteristics and a formation damage model can be used to incorporate these variations into a history matching process for the characterization of reservoir systems which can also be used for accurate prediction of future performance. Recent literature surveys have had various arguments and debate about if formation damage is considered more detrimental for the vertical wells or for the horizontal ones. However, the fact still remains that in both cases, the production loss due to formation damage is significant.