Alternative Weighting Material
Authors: Walker C.O. 
Published: December 2007

Summary
Texaco Inc.'s experiences with mined iron oxide used as an alternative weighing material for barite are presented. This paper discusses laboratory studies made before field application of this product. Also included are test results of a SWACO choke assembly to simulate field conditions.

Laboratory Testing Compliance With APE Specifications
The materials tested were mined iron oxide, iron titanate, refined iron oxide, and two product blends. Blend A is a mixture of 80% 4.0-gravity barite and 20% mined iron oxide. Blend B consists of 73% 4.0-gravity barite and 27% mined iron oxide. These compounds were tested according to API barite procedures. All samples met the specifications (Table 1). Note that one possible advantage the iron compounds have over barite is their high specific gravities. The use of these materials potentially could reduce costs. Less material would be required to increase weight: a lower-solids fluid would be possible that could decrease drilling time; and fewer solids would be incorporated into the mud system to affect flow properties negatively, thus requiring fewer chemical treatments. Also, the properties negatively, thus requiring fewer chemical treatments. Also, the iron materials reportedly are less prone to reduction in size through attrition than barite. If these iron-containing materials are accepted by the industry as replacements for barite, it appears that specifications similar to those currently used for barite would have to be devised. Particle-size distribution within a given product and the presence of contaminating ions are still important criteria that must be considered.

Effect on Mud Rheology
The performance of these weighting materials was compared to barite in a lignosulfonate mud. A tophole mud was treated with 6 lbm/bbl [17.1 kg/m3] of lignosulfonate and the pH was adjusted to 10.5 with caustic soda before aging overnight at 150F [65.5C]. The base mud was then cooled, weighted up to approximately 15 lbm/gal [1785 kg/m3] with the various weighting materials, and again aged in a roller oven for 16 hours at 150F [65.5C]. The samples were then removed from the oven, cooled to room temperature, and the pH readjusted to 10.5, if necessary, before testing. In the 15-lbm/gal [1785-kg/m3] samples, blend B had a higher yield point and gel strengths than any of the other materials tested (Table 2). The distortion of mud rheology with this blend is most probably a result of the barite used since the sample containing mined iron oxide and the sample containing Blend A performed well. Blend B is composed of 73 wt% of 4.0-specific-gravity barite and 27% mined iron oxide. The poor-quality barite used in this blend probably contains a substantial amount of fine solid particles that could act as contaminants when a mud is weighted up.

Abrasive Character
At this time there is no API-approved method for determining the abrasiveness of oilfield materials. In lieu of this, preliminary laboratory tests were made with a Waring Blendor to determine the relative abrasiveness of each of the products tested. In each case, a mixing blade was weighed and installed. The fluid to be tested was placed in the blender, and the mud was sheared for 20 minutes at approximately 12,000 rpm [200 rev/s]. The weight loss of the blade was then determined and the percent weight loss was calculated. It was assumed that the greater the percent weight loss was calculated. It was assumed that the greater the weight loss, the greater the abrasiveness of the fluid. In each case the unblended iron compounds exhibited more abrasiveness than either barite or the blended materials (Table 3).

Simulated Field Application
In actual field operations one of the most severe conditions, from an abrasion standpoint, occurs when a well is put on the choke to help control a "kick."
JPT
p. 2158