Successful Diversion of Acid Stimulation in Water Producing Oil Wells: Case Histories, Nisku Formation, Central Alberta

Abstract

Stimulation of carbonate oil and water producers with acid can result in an uneconomical increase in water production. This is the result of poor placement of the treatment stimulating the water-bearing rock preferentially to the oil. Typical treatments in the Nisku use selective tool isolation of zones to achieve the required placement control. The operational costs associated with this type of treatment make it difficult to perform economical stimulation treatments on marginal wells. Acid treatment of such a well, using only a diverting agent for control of placement, risks increased operational costs associated with possible increased water production and, therefore, the reduction of a marginal well to an uneconomical one. A unique diverting material formed by the reaction of an aqueous solution of dibasic acids and rosin esters, with divalent cations, present in formation water or spent acid allows the selective stimulation of only the oil-producing zones. The reaction product is an oil-soluble precipitant. The oil-soluble nature of the diverter base materials and the resultant precipitants makes the likelihood of the precipitant forming in the oil zones remote; however, in the event of such a result, the produced oil will remove the precipitant. Presented are laboratory tests describing the effectiveness of the oil-soluble material to divert acidic fluids away from water flows. Also, demonstrated is the subsequent cleanup of this oil-soluble diverting material using produced oil. In addition, several case histories from the Wayne area of Alberta, Canada are included. The results of these treatments demonstrate that control of acid placement in water-cut producers results in a considerable increase in oil production while minimizing the effect on water cuts. A significant reduction in the operational costs associated with switching from the selective tool isolation technique to the one described above is also demonstrated. Introduction Acidizing has long been accepted as a means of increasing production from oil and gas wells. A major problem with many wells is the increased production of water along with the hydrocarbons1,2. The scenario is even more accentuated if placement control is not used during the stimulation treatment. Placement control methods vary around the world2,3,4.These methods involve use of diverting materials5, foams6, mechanical isolation or the use of varying injection rate7. The production of water is an added cost to the operator in the form of disposal, treating and/or the possibility of scale formation in the wellbore. For example, a 6m3/day increase in water production is approximately equivalent to a $2.50/day ($ Canadian) increase in operational costs. A 180m3/day increase would mean a $75/day increase. Therefore, the extra revenue from an increase in oil production could potentially be offset by the increase in operational expense of the additional water. The water in the Wayne area is highly corrosive which, in turn, increases the cost of disposal. In Table 1, the costs are broken down into catagories, including chemicals to deal with the corrosivity. The primary difficulty with treatments that have had the least amount of success has been the inability to place the acid where it will do the most good. A correlation can be made between unsuccessful stimulation treatments (in terms of water inflow increases), and the inability to effectively place acid. This paper is focused on treatments that were conducted in the Nisku formation in south central Alberta (Figure 1).

A Novel Technique to Acidize Multilateral Open Hole ER Horizontal Wells Drilled in Carbonate Formations

“…These include foam, 14,15 gelled, and in-situ gelled acids. [16][17][18] Nasr-El-Din and co-workers have raised several concerns regarding the use of polymer-based acids in tight formations, especially in sour environments. 19,20 Polymerbased diverters can cause formation damage in some cases.…”

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“…These include foam, 14,15 gelled, and in-situ gelled acids. [16][17][18] Nasr-El-Din and co-workers have raised several concerns regarding the use of polymer-based acids in tight formations, especially in sour environments. 19,20 Polymerbased diverters can cause formation damage in some cases.…”

Acidizing: A Small Change in Recipe, A Big Improvement in Production.

“…As such, acid stimulation had long been accepted as one of the methods to increase production in West Lutong wells as recommended by G. Henderson et al (2002).However a major problem associated with these wells is the increased production of water along with hydrocarbon. Therefore, this scenario is even more accentuated if diversion is not been used during stimulation treatment.…”