Acidizing: A Small Change in Recipe, A Big Improvement in Production.
Authors: Wahib Suhaila , Nizam Effendi Shahrul , Choo Der Jiun , Chun Seng Wong , Ramnarine Ronald Peter 
Published: February 2011

Abstract The energy global demand is predicted to increase by an estimated 2-4 percent annually over the next decade. However, production decline from mature field such as West Lutong is estimated about 5-6 percent annually. Therefore, acid stimulation provides a cost effective method of increasing production and accelerating reserves. Previously, acid stimulation in West Lutong had always been plagued with "consistent?? increasing average water cut of 5-10% due to accidental stimulation of water zones and "inconsistent?? skin removal ranging from 30% to 75%. As part of continuous stimulation improvement effort, historical stimulation recipe and post treatment had been analyzed. An innovative "nitrified acid stimulation with refined oil soluble resin diverter recipe?? had been chosen for the pilot implementation. Pre calculated volume of nitrogen is mixed on the fly with acid at surface before pump into the formation. Due to low solubility, nitrogen commingled with acid assumes state of compressed gas bubbles energizes the injected fluids. When the well pressure is relieved the gas bubble expands, greatly aiding the injected fluids into formation returning to surface. As a result, it increases radial penetration for given acid volume due to additional volume of gas. Oil soluble resin diverter stages had been reduced from two to one since the success rate is independent of stages pumped and for operation simplicity. The pilot job at Well # A had successfully increased oil production about 537% (instantaneous gain of 1289 bopd) contributed by 87% skin removal while reducing the water cut by 5 % since July 2009. In addition, 0.3 mmstb of reserves is expected to be monetized. This paper shares a detailed case study of Well # A in term of candidate's selection, nitrified acid stimulation recipe, execution, lesson learnt, and future recommendations. It is evident that nitrified acid stimulation with oil soluble resin diverter is feasible and economic for high water cut acid stimulation. Introduction West Lutong is a mature field with 8 rounds of field development campaigns and close to 40 years of production in Sarawak waters, Malaysia.Based on historical build up and flowing gradient surveys, it had confirmed damaged skin in most of the wells in West Lutong. The damaged skin was suspected due to fines migration with increasing water cut in gravel packed wells. In addition, wireline run had recovered severe scale and minor wax/asphaltene in the past. 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. Previously, foam diverter was used as diversion means from 2000 to 2007 for non selective acid stimulation due to inavailability of proven selective acid stimulation technology. Oil soluble resin diverter had been introduced in 2007 for selective acid stimulation and had been used since due to successful pilot in 2007 with drop of around 5-10% water cut.