Formations damage

Much has been written over the years on the subject of formation damage. For background on formation damage, the best place to start is with the landmark paper by Krueger. Another occasionally forgotten but very important work is that of Maly. This chapter is concerned with formation damage issues as they relate to acid treatment design. 

To assess formation damage, it is necessary first to understand the skin term in the equation, derived from Darcy s law, that defines well production rate and then to understand its effect on production rate. Well production rate, defined by 

B is the formation volume factor—that is, reservoir volume/production volume (RB/STB) 

Importantly, production rate, q, is directly proportional to permeability, k, and inversely proportional to skin, s. Along with reservoir quality, these two variables, k and r, are of greatest importance in stimulation design. The combination of low permeability and high skin makes for a very unproductive well. These two factors must be understood and defined before considering a well stimulation procedure. 

Skin is a mathematical representation of the degree of damage present. It can be represented, qualitatively, by the Hawkins equation, as follows  

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Better well control allows at-balance or even underbalanced drilling, resulting in higher penetration rates and reduced potential for formation damage. 

Perhaps the greatest stimulus for the development of such tools has been the proliferation of high angle wells in which deviation surveys are difficult and wireline logging services are impossible (without some sort of pipe conveyance system), and where MWD logging can minimise formation damage by reducing openhole exposure times. 

Keywords production decline, economic decline, infill drilling, bypassed oil, attic/cellar oil, production potential, coiled tubing, formation damage, cross-flow, side-track, enhanced oil recovery (EOR), steam injection, in-situ combustion, water alternating gas (WAG), debottlenecking, produced water treatment, well intervention, intermittent production, satellite development, host facility, extended reach development, extended reach drilling. 

No loss-of-circulation problem No formation damage Very high penetration rate Low bit costs Low water requirement No mud requirement No ability to counter subsurface pore pressure problems Little ability to carry formation water from hole Hole erosion problems are possible if formations are soft Possible drill string erosion problems Downhole fires are possible if hydrocarbons are encountered (gas only) Specialized equipment necessary... 

In most air and gas drilling operations, open-hole well completions are common. This type of completion is consistent with low pore pressure and the desire to avoid formation damage. It is often used for gas wells where nitrogen foam fracturing stimulation is necessary to provide production. In oil wells drilled with natural gas as the drilling fluid, the well is often an open hole completed with a screen set on a liner hanger to control sand influx to the well. 

A reduction in permeability around the wellbore mainly caused by contact with drilling fluid (formation damage]. 

Polyacrylates are often added to drilling fluids to increase viscosity and limit formation damage. The filter-cake is critical in preventing reservoir invasion by mud filtrate. Polymer invasion of the reservoir has been shown to have a great impact on permeability reduction [98]. The invasion of filtrate and solids in drilling in fluid can cause serious reservoir damage. 

Filter-cakes are hard to remove and thus can cause considerable formation damage. Cakes with very low permeability can be broken up by reverse flow. No high-pressure spike occurs during the removal of the filter-cake. Typically a high-pressure spike indicates damage to the formation and wellbore surface because damage typically reduces the overall permeability of the formation. Often formation damage results from the incomplete back-production of viscous, fluid loss control pills, but there may be other reasons. 

Sometimes it may become necessary to shut-in a gas well when the demand for gas is low. In such instances, the well is shut-in for an indefinite period, after which it is reopened and production is resumed. It often has been found that the production rate of gas from the reopened well is substantially less than it was before the well was shut-in. During production, the inner wall of the production tubing will be coated with a film of condensed freshwater because of the geothermal gradient. This water flows down when production is interrupted and can cause formation damage. This may occur because clays are normally saturated with brine water and not with freshwater. This swelling can be prevented with the injection of some additive, for example, sodium chloride, potassium chloride, calcium chloride, or an alcohol or a similar organic material [1853]. 

However, production engineers have been reluctant to use particle bridging because of the possibility of particle transport into the formation, resulting in formation damage and/or costly and often ineffective stimulation treatments. A particle bridging fluid has been developed that quickly and effectively controls fluid loss in a wide range of permeabilities and pore diameters [916]. 

Formation damage caused by clay migration may be observed when the injected brine replaces the connate water during operations such as water-flooding, chemical flooding including alkaline, and surfactant and polymer processes. These effects can be predicted by a physicochemical flow model based on cationic exchange reactions when the salinity decreases [1665]. Other models have also been presented [345,1245]. 

Additives that assist the creation of a fracture include viscosifiers, such as polymers and crosslinking agents temperature stabilizers pH control agents and fluid loss control materials. Formation damage is reduced by such additives as gel breakers, biocides, surfactants, clay stabilizers, and gases. 

F F Chang and F Civan. Practical model for chemically induced formation damage. 7 Petn / Sci Eng, 17(1-2) 123-137, February 1997. 

W. C. Chin. Formation invasion With applications to measurement-while-drilling, time lapse analysis, and formation damage. Gulf Publishing Co, Houston, 1995. 

C. K. Deem, D. D. Schmidt, and R. A. Molner. Use of MMH (mixed metal hydroxide)/propylene glycol mud for minimization of formation damage in a horizontal well. In Proceedings Volume, number 91-29.4th CADE/CAODC Spring Drilling Conf (Calgary, Canada, 4/10-4/12) Proc, 1991. 

J. L. Elbel, R. C. Navarrete, and B. D. Poe, Jr. Production effects of fluid loss in fracturing high-permeability formations. In Proceedings Volume, pages 201-211. SPE Europe Formation Damage Contr Conf (The Hague, Netherlands, 5/15-5/16), 1995. 

B. Evans and S. Ali. Selecting brines and clay stabilizers to prevent formation damage. World Oil, 218(5) 65-68, May 1997. 

W. W. Frenier, C. N. Fredd, and F. Chang. Hydroxyaminocarboxylic acids produce superior formulations for matrix stimulation of carbonates. In Proceedings Volume. SPE Europe Formation Damage Conf (The Hague, Netherlands, 5/21-5/22), 2001. 

J. Gulbis, M. T. King, G. W. Hawkins, and H. D. Brannon. Encapsulated breaker for aqueous polymeric fluids. In Proceedings Volume, pages 245-254. 9th SPE Formation Damage Contr Symp (Lafayette, LA, 2/22-2/23), 1990. 

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