An Experimental Study of the Swelling Behaviour of Mudrocks In the Presence of Water-based Mud Systems
Authors: Amanullah Md. , Marsden J.R. , Shaw H.F. 
Published: March 2010

Abstract
Swelling behaviour of three different mudrocks (London Clay, Oxford Clay and Fuller's Earth) of varying composition, geological ages, degree of lithification, organic matter and. expandable clay contents have been investigated in the presence of water-based mud systems (KCIICMC, Polyamine, Polyamine, + glycol and PHPA + glycol) plus water to identify the factors controlling the swelling behaviour of shales. The results show, that the degree of swelling is not directly correlatable with the proportion of swelling clay minerals in the mudrocks. Based on the experimental results, the swelling behaviour of the mudrocks depends on the composition, geological history and degree of, lithification of the mudrocks. Well lithified smectite-rich mudrock was found to show less swelling than less well lithified mudrocks with lesser amounts of swelling clays. Organic matter rich mudrock showed a greater degree of swelling than those with little organic matter, suggesting that organic matter may also influence the swelling behaviour. This investigation pro' vi des a better understanding of the swelling behaviour of mudrocks by introducing the concept of the combined action of physico-chemical and mechanical swelling processes.

Introduction
Swelling behaviour of different rock-fluid combinations is very important in predicting different drilling problems and determining the proper mud system to stabilize the troublesome shaley borehole environment. This characteristic of rocks is a complex function of the rock type and the mud systems and is one of the main causes of sloughing, heaving, caving and progressive hole enlargement problems in the shaley section of a borehole environment, 3). Swelling causes easy detachment of clays or shale particles both from the borehole wall and the cuttings, incorporating into the mud system as additional solid materials, and thus produces considerable changes in the rheological properties of the mud, increasing risk of differential sticking, induced fracturing and mud treatment cost.
It is widely believed that smectite clay content of the mudrocks is the main culprit for excessive swelling of shale materials. In the context of drilling, this criterion of shale swelling is often unable to explain the unusual swelling behaviour of some mudrocks having little or no swelling clays. On the other hand it was found that some smectite' rich shale formations(2) have very little swelling potential compared to shales having low expandable clay content. his illustrates the role of other factors in controlling the swelling behaviour of mudrocks. Bearing this in mind an investigation was carried out to determine the factors governing the swelling behaviour of mudrocks.
TABLE 1: Composition of the clay fraction &lt;2 micron) and TOC of the mudrocks.
Three different mudrocks and four different drilling mud systems plus fresh water were used for the study. The mudrocks are London Clay representing the Upper Tertiary mudrock, Oxford Clay representing the Upper Jurassic mudrock and Fuller's Earth representing the Lower Tertiary mudrock encountered in the North Sea. Some commonly used water-based muds such as KCI/CMC, Polyamine, Polyamine + glycol, water and a newly designed water-based mud system were used to assess the swelling behaviour and factors governing the behaviour of the mudrocks.