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→Near wellbore restrictions
==Near wellbore restrictions==
==Near wellbore restrictions==
[[file:production-problems_fig2.png|thumb|{{figure number|2}}Effect of partial penetration on flowing pressure.]]
Restrictions can occur in the formation or within the wellbore that can cause a decrease in oil or gas production. These restrictions are a result of changes in the formation or fluid properties around the wellbore, chemical reactions within the formation or the wellbore, mechanical problems, or inadequate completion techniques.
Restrictions can occur in the formation or within the wellbore that can cause a decrease in oil or gas production. These restrictions are a result of changes in the formation or fluid properties around the wellbore, chemical reactions within the formation or the wellbore, mechanical problems, or inadequate completion techniques.
===Partial penetration===
===Partial penetration===
If only a portion of the productive formation has been drilled or perforated, this is referred to as ''partial penetration''. Gas or oil that is flowing from the reservoir to this limited area will cause large pressure gradients near the wellbore. Fluid flow within the pores may also reach turbulent velocity, thus generating additional pressure drop and reducing the productivity of the well. Studies by Harris<ref name=pt09r13>Harris, M. H., 1966, The effect of perforating on well productivity: Journal of Petroleum Technology, April, p. 518.</ref> and Odeh<ref name=pt09r19>Odeh, A. S., 1968, Steady-state flow capacity of wells with limited entry to flow: Society of Petroleum Engineers Journal, March, p. 43.</ref> on partial penetration have concluded that the effects are similar to skin damage around the wellbore (that is, an altered [[permeability]] that is lower than the reservoir permeability). To overcome these effects, the well must be deepened to expose all of the productive interval or more perforations must be added to expose additional reservoir to flow. Figure 2 illustrates the effects of partial penetration on pressure drop around the wellbore.
If only a portion of the productive formation has been drilled or perforated, this is referred to as ''partial penetration''. Gas or oil that is flowing from the reservoir to this limited area will cause large pressure gradients near the wellbore. Fluid flow within the pores may also reach turbulent velocity, thus generating additional pressure drop and reducing the productivity of the well. Studies by Harris<ref name=pt09r13>Harris, M. H., 1966, The effect of perforating on well productivity: Journal of Petroleum Technology, April, p. 518.</ref> and Odeh<ref name=pt09r19>Odeh, A. S., 1968, Steady-state flow capacity of wells with limited entry to flow: Society of Petroleum Engineers Journal, March, p. 43.</ref> on partial penetration have concluded that the effects are similar to skin damage around the wellbore (that is, an altered [[permeability]] that is lower than the reservoir permeability). To overcome these effects, the well must be deepened to expose all of the productive interval or more perforations must be added to expose additional reservoir to flow. [[:file:production-problems_fig2.png|Figure 2]] illustrates the effects of partial penetration on pressure drop around the wellbore.
[[file:production-problems_fig2.png|thumb|{{figure number|2}}Effect of partial penetration on flowing pressure.]]
===Scales and precipitates===
===Scales and precipitates===