New Method of Measuring Gel Strength of Clay-Based Drilling Fluids and the Determination of their Long-Term Gelation Behaviour
Authors: Ghofrani R. , Strahl F.G. , Bosch V. 
Published: March 2010

Abstract
For avoiding environmental pollution, predominantly water based drilling fluids are employed in drilling operations. If these drilling fluids contain clays as structural agents, contamination by electrolytes or exposure to elevated temperature, or both, can result in more or less severe flocculation and thickening; this phenomenon can cause serious problems during the resumption of circulation, especially after long round trips, as well as swab and surge. For this reason, it is especially important in practice to be capable of predicting the long-term gelation behavior of the drilling fluid on the basis of short-term gel strength measurements.
In the present study, it has been shown that reproducible gel data can be obtained only in exceptional cases on water-based clay drilling fluids exposed to electrolytes or elevated temperature, or both, with the use of measuring instruments standardized in conformance with API (such as the Fann V-G Meter, model 35).
On the other hand, it has been shown that arbitrarily high gel strength values can be determined reproducibly with the ITE instrument for measuring gel strength of clay suspensions (GMT), because the apparent wall slip is eliminated. On the basis of short-term gel strength values measured with the ITE GMT, the long-term gelation behavior of water-based clay drilling fluids can be predicted with sufficient accuracy by means of the analytical method introduced by Garrison in 1938; this is of particular interest especially after lengthy round trips.

Introduction
For reasons of environmental protection, water-based drilling fluids are now being used to an increasing extent in drilling operations, even under difficult drilling conditions. As a rule, clays are employed as structural agents in these drilling fluids. For stabilizing the clay against electrolytes or elevated temperatures, or both, protective colloids are added. As a result, the polymers impose their flow behavior on the clay; that is, the differential (true) viscosity of the clay suspensions treated with polymers decreases as a function of the shear stress. Consequently, the clay is then responsible only for the value of the yield point in these systems.
If water-based drilling fluids are exposed to electrolytes or elevated temperatures, or both, the protective action of the added polymers may be inhibited; as a result, the flow and gelation behavior of the clay may break through. Especially during long round trip times, the severely gelled drilling fluid can then cause problems with the pumping pressure necessary for resuming the circulation as well as with swab and surge. Hence, there is a need for reliable measuring methods and analytical techniques for predicting the long-term development of the gel strength.
In the present study, the standardized API methods for measuring gel strength are first examined for reproducibility of the results. If appropriate measures prove necessary, suitable methods are developed and investigated for measuring the gel strength. Finally, the analytical method of Garrison(1) is examined for its applicability in calculating the long-term gel strength from the results of short-term measurements.