Factors Affecting Downhole Scale Deposition: North Dakota Bakken Formation
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 140977, ’Field Study of the Physical and Chemical Factors Affecting Downhole Scale Deposition in the North Dakota Bakken Formation,’ by Lawrence M. Cenegy, SPE, Clyde A. McAfee, SPE, and Leonard J. Kalfayan, SPE, Hess Corporation, prepared for the 2011 SPE International Symposium on Oilfield Chemistry, The Woodlands, Texas, 11-13 April. The paper has not been peer reviewed.

Oil recovery from the North Dakota Bakken formation has increased nearly 100-fold over the last 5 years. For one North Dakota operator with 150 Bakken producing wells, 22 of the wells have experienced at least one event of severe calcium carbonate scaling in the pump and production tubing, leading to well failure. A study showed that 82% of those wells failed during early production (i.e., less than 20,000 bbl of water produced and 2 years production); after this point, failures became increasingly rare. This rate correlated with transient alkalinity spikes in the water analyses and was attributed to fracturing-fluid flowback. Simulated blending of fracturing and formation waters demonstrated that during this period, it was most important to maintain a high level of scale inhibitor.

Introduction

The Bakken formation is within the Williston basin in western North Dakota and northeastern Montana in the USA and in southern Saskatchewan and southwestern Manitoba in Canada, as shown in Fig. 1. Discovered in 1953, the formation contains three key layers: an upper shale member (approximately 25 ft thick) that separates it from the Mississippian Lodgepole formation, a middle sandstone and silty-sandstone member (approximately 35 to 90 ft thick), and a lower shale member (approximately 50 ft thick) that separates it from the Devonian Three Forks formation. While the upper and lower shale members are rich in organic matter, the middle sandstone layer is most often produced economically because of its greater porosity. Bakken production occurs from approximately 8,000- to 11,000-ft vertical depth, with temperatures up to 240°F and pressures of 5,500 to 6,000 psi. The oil gravity is 39 to 46°API.

Background

Horizontal drilling is used to exploit reserves because the middle member is a relatively thin layer. Wells completed with 10,000-ft laterals are common. This method allows access to more oil-producing zones; however, the low permeability of the Bakken middle member requires effective hydraulic fracturing for production. Initially, the opera-tor conducted only “scour frac” operations, in which a single, high-rate fracturing treatment was pumped into the horizontal open hole without controlled placement. The result was cracking of the formation at its weakest point rather than a distribution of multiple propped fractures along the wellbore. Often, the result was excessive fracture-height growth and water incursion from other formations.

Newer fracturing technology enables pinpoint multizone fracturing of the horizontal section. Pinpoint fracturing uses openhole packers and ball-actuated sliding sleeves to isolate targeted intervals. The operator applies such a system to fracture a mini-mum of 18 zones per lateral in a continuous operation.