Prudhoe Bay Field Study - Return Fluid Analysis
Authors: Almond Stephen W. , Brady Jerry L. , Underdown David R. 
Published: December 2007

Summary.
Monitoring ionic content of returned fluids after various acidizing treatments gives insight into actual chemical acid-spending processes that occur downhole. In this study, laboratory tests and field treatments were performed, and the resultant dissolution processes were monitored. Results show that the high siderite concentration (6 to 12 %) in the Sadlerochit formation accounts for most (60 to 70%) of the hydrochloric acid spending, as well as 10 to 15% of carbonate spending and 10 to 15% of clay dissolution during HF acid treatments. The HF spending was primarily on quartz/ chert in the laboratory. Field results, however, indicate that the secondary clay dissolution process was the primary HF spending reaction.

Introduction
The Prudhoe Bay field was discovered in 1968, and subsequent drilling confirmed that it was the largest oil and gas field in North America. The Prudhoe Bay Unit became effective on April 1, 1977, and began production on June 19, 1977, after completion of the Trans-Alaska Pipeline System. Since then, more than 750 wells have been completed in the Sadlerochit formation. Initial stimulation treatments were based on calcium carbonate scale removal and subsequent inhibitor squeezes. These treatments were quite effective in a high percentage of wells where scaling problems had been identified. In late 1984 and early 1985, HF acid treatments were started on the Sadlerochit formation to stimulate wells that did not respond to the established EDTA scale treatments. Since then, several HF acid treatments have been performed successfully. However, a greater understanding of downhole chemistry was needed to improve treating response even further. Several authors have presented laboratory studies on HF acidizing and the actual reactions occurring in various core types. Gdanski and Peavy presented a study on well return analysis and a method for calculating the actual downhole reaction with the specific ionic concentrations from fluids returned after a retarded-HF acid treatment.
This paper describes a comprehensive study on acidizing in Zones 1B and 4 of the Sadlerochit formation. Mineralogy, solubility, oil, and acidizing chemistry studies were performed in the laboratory with Sadlerochit core samples to give an idea of the products that would be expected upon acidizing fluid return. Also discussed is the field study of several acidizing treatments and analyses of the actual returned fluids.

Experimental Procedures

Mineralogy.
Mineralogy of the various core samples was analyzed by X-ray diffraction (XRD). An X-ray powder diffractometer was used on a bulk powder sample from Well 12–3.

Acid Solubility Tests.
In these tests, 1.00 g of disaggregated sample (from Well 12–3) was placed into 100.0 nit of either 15% HCl or 12 % HCl/3 % HF acid for 2 hours at 150 degrees F [66 degrees C] under a N2 gas blanket. Solubilities were determined by weight loss.
Total iron content was determined by boiling the disaggregated sample with HCl acid containing H2O2. Extracted iron was determined by titration with dichromate.

Dissolved-Solids Analysis.
The supernatant fluids from the acid solubility tests were analyzed for dissolved solids contents. Cationic concentrations of Ca, Mg, Si, Ba, Al, Sr, Na, Fe, and K were determined from atomic absorption measurements. Total Fe concentrations on Well 12–3 samples were obtained from atomic absorption measurements; Fe3+ and Fe2+ concentrations were obtained spectrophotometrically. All concentrations are expressed in milligrams per liter.

Returned Fluid Collection and Analysis.
An elaborate collection scheme was implemented to perform a return fluid analysis. Samples of acids used to treat the formation or tubing were collected before the job to provide a known acid concentration and ionic content. When possible, a sample of the produced formation water was collected for ionic reference of the makeup fluid. Ions for which fluid was evaluated were Cl, SO4, Si, Fe (total Fe, Fe2+, and Fe3+), Ca, Al, K, Mg, Sr, Ba, Na, and F. Analyses for the specific ions were performed in a laboratory on the North Slope. In addition to the specific ion concentrations, a date, collection time, production rate, cumulative water production, water cut, pH, specific gravity, and live acid concentration of the returned samples were recorded.

Mineralogy
Return analyses-from two of the four production zones in the Sadlerochit formation (Zones 1B and 4, with the emphasis on Zone 4 wells) was completed.
JPT
P. 466⁁