Evaluation of Low-solids Mud Reological Behavior during Drilling Shale Formation and Their Effect on the Pay Zone Productivity

Abstract

The present study deals with the evaluation of rheological behavior of low-solids mud during drilling shale formation and its effect on the pay zone productivity. This study involves the preparation of shale samples (shale formation) required for the shale sloughing tests and preparation of plug samples (reservoir formation) required for the saturation tests. The shale analyzed by X-ray diffraction showed that the main constituents are montmorillonite, small amounts of kaolinite and smectite, with a little bit of illite recorded in a few samples. Also, the low-solids base mud was prepared in form unweighted then weighted by barrite and the rheological properties (density, apparent viscosity, plastic viscosity yield point, gel strength, and thixotropy) were determined for the two types; the weighted low-solids base mud was then circulated with the shale for 12, 24, 36, 48, and 60 hr using the roller oven device to determine the sloughing ratio of the shale and the rheological properties after circulation. This circulation showed that the shale sloughing ratio increased to more than 90%. Afterward, the core samples (reservoir rock) were prepared in the form of plugs according to the test requirements and their physical properties, including porosity, permeability, and resistivity, were determined through clean samples, and after saturation by low-solids mud and shaly low-solids mud, a petrographic study was carried out for the clean and saturated samples by making thin sections to show the change in the porosity. From these it was concluded that the rheological behavior of the low-solids mud during drilling shale formation affected caused formation damage and had an effect on the pay zone productivity. The recommendation for minimizing the shale affect and formation damage is to add specific chemicals to the low-solids mud system during the drilling.