The Role of Bound Water and Capillary Water in the Evaluation of Porosity in Reservoir Rocks

Nadia Pallatt & David Thornley
BP Research Centre, Sunbury-on-Thames, TW16 7LN

Abstract:
Knowledge of the quantity and distribution of the different types of water contained within the interconnected pore volume of a reservoir rock is fundamental to formation evaluation. Three main water types have been identified as bound, capillary and free waters. Bound water includes both water held to the net-negatively charged mineral surfaces, and the water of hydration associated with the mineral charge balancing counter-ions. Capillary water is immobile water held by capillary forces in regions of microporosity, for example dead end pores and pendular rings. Free water is water that is able to flow under a given pressure gradient.

A new laboratory technique, thermogravimetry/evolved water analysis TG/EWA, has been used to quantify these different water types. Bound/capillary water relationships are presented for core taken from three different North Sea reservoirs that exhibit very different clay mineralogy. Berea sandstone was included in the study. The data have been integrated with measurements of porosity, surface area, mercury porosimetry, SEM and XRD to explain the influence that rock structure and mineralogy have on the quantity and distribution of water types. All the rocks studied contain much more capillary held water than bound water. The amounts held are strongly dependent on the type of clay mineral that predominates, along with its morphology and location within the pore system. This study has shown that reservoir rocks that contain chlorite as the major clay mineral can hold up to 10 times more capillary water than bound water.

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About the Society of Core Analysts

Objectives
The name of this organization is the Society of Core Analysts (SCA) and was founded in 1986. This organization is a Chapter-at-Large of The Society of Petrophysicists and Well Log Analysts (SPWLA) and is established with two objectives:

• To promote the aims, purposes, and membership of the SPWLA which is a non-profit scientific organization.
• To serve the interests of all persons who use or obtain reservoir evaluation information from rock and core samples.

As a guide for fulfilment of the first objective, this Chapter stands responsible to, and subject to, the rules and regulations of the SPWLA.

As a means toward the second objective this Chapter will sponsor technical speakers, projects, and presentation of papers dealing with formation evaluation. This Chapter will hold membership open to all core analysts, log analysts, geologists, geophysicists engineers, service company personnel and all others interested in the study of rocks and their interstitial fluids. This Chapter will also actively encourage membership and participation in the activities of the SPWLA.

Website: www.scaweb.org