EDTA Removes Formation Damage at Prudhoe Bay
Authors: Shaughnessy C.M. , Kline W.E. 
Published: December 2007

Summary
Formation damage has been observed in some wells of the Prudhoe Bay field. Laboratory and field testing confirmed that the primary cause of damage was the buildup of calcium carbonate scale either in the perforation tunnels or in the formation sandstone near the wellbore. Conventional acid treatments could dissolve this scale, but scale reprecipitation from the spent acid caused rapid productivity decline. A scale-removal treatment with Na2H2, ethylenediaminetetraacetic acid (EDTA) has been developed that can effectively dissolve scale and chelate the dissolved metal ions. Chelation of the dissolved scale prevents scale reprecipitation. Of the 25 Prudhoe Bay wells acidized with EDTA through June 1982, 19 responded with significant productivity increases. In 17 of these wells, productivity has been sustained after treatment.

Introduction
Soon after startup of the Prudhoe Bay field, a fraction of the completed wells exhibited productivity-decline rates significantly greater than the 10 to 20%/year anticipated by geologic and reservoir studies. A history of the well-damage problem at Prudhoe Bay and the response by the working-interest owners has been described previously by Braunston. Although the reason for these high declines was not known initially, it was recognized that a successful response would require a coordinated program of engineering, field, and laboratory activity. A Well Damage Study Group was formed with representatives from the three major owners-Sohio, ARCO, and Exxon. A review of drilling, completion, and well test data revealed two groups of high-decline wells. In one small group of wells, the onset of high decline rates could be traced directly to formation invasion by calcium-chloride brine. This brine was used as a completion/work over fluid at Prudhoe Bay because of its density and freezing-point characteristics. In the second, larger group, there had been no formation invasion by calcium chloride. For these wells, the damage occurred solely from the production of reservoir fluids. Even though the wells produced only trace amounts of water, the most likely source of damage was some type of scale. Diagnostic field treatments supported the hypothesis that damage was caused by near-wellbore scale. As described in detail by Braunston, the well productivity lost to damage could be recovered by reperforating the original interval. This result showed the formation damage to be in the perforation tunnels or very close to the wellbore. Carbonate-scale damage was indicated when productivity increases were obtained either by matrix acidizing with HCl or by simply spotting HCl in the wellbore across the perforations. Unfortunately, the initial productivity increase from any of these techniques was very short-lived. The response of one well to repeated hydrochloric acid treatment is shown in Fig. 1. A high decline rate was observed immediately following each treatment with HCl. Laboratory studies to address this high-decline problem proceeded in close cooperation with the engineering and field activities. Experimental work confirmed that calcium carbonate scale is the primary cause of damage at Prudhoe Bay. Nondamaging work over fluids were formulated by employing mixtures of sodium bromide and sodium chloride as a substitute for calcium chloride.
JPT
P. 1783^