Gj van Tonder

Estimation of the sustainable yields of boreholes in fractured rock formations

Abstract The simplest way to derive an estimate for the sustainable yield of a borehole is to study the behaviour of drawdowns observed during a hydraulic (also known as a pumping test) of the borehole, through an appropriate conceptual model. The choice of this model is probably the most difficult choice that the analyst of such a hydraulic test has to make, since a wrong model can only lead to the wrong conclusions and failure of the borehole. This paper discusses a semi-analytical and two numerical methods that can be used to simplify the analyses of hydraulic tests in fractured rock formations. The first method, called the Method of Derivative Fitting (MDF), uses a new approach to identify the conceptual model needed in such analyses. This is achieved by characterizing the various flow periods in fractured rock aquifers with numerical approximations of the first logarithmic derivative of the observed drawdown (the derivative of the drawdown with respect to the logarithm of the time). Semi-analytical expressions are used to estimate the influence that boundaries may have on the observed drawdown and the sustainable yield of a borehole — the rate at which a borehole can be pumped without lowering the water level below a prescribed limit. An effort has also been made to quantify errors in the estimates introduced by uncertainties in the parameters, such as the transmissivity and storativity, through a Gaussian error propagation analysis. These approximations and the MDF, called the Flow Characteristics Method (FCM) have been implemented in a user-friendly EXCEL notebook, and used to estimate the sustainable yield of a borehole on the Campus Test Site at the University of the Orange Free State. The first numerical method, a two-dimensional radial flow model, is included here because it allows the user more freedom than the FCM, although it requires more information. One particular advantage of the method is that it allows one to obtain realistic estimates of the storativity and transmissivity of Karoo aquifers in particular, which is required in the estimation of the sustainable yield of a borehole. There is no doubt that a three-dimensional numerical model, the second numerical method discussed here, is the best method with which to analyse a hydraulic test in a fractured aquifer. The method was consequently used to evaluate the accuracy of the implementation of the MDF in the Excel notebook and its application to the borehole on the Campus Test Site. The good agreement between the sustainable yield estimated with the three-dimensional numerical model and the FCM indicates that the FCM can be used with confidence to estimate the sustainable yields of boreholes in fractured media.

ESTIMATION OF NATURAL GROUNDWATER RECHARGE IN THE KAROO AQUIFERS OF SOUTH AFRICA

A 3-year project, to study the natural groundwater recharge of aquifers in the semi-arid Karoo formations of South Africa, was undertaken. Two typical Karoo aquifers, at Dewetsdorp and De Aar, were selected for study purposes. Data were collected from both the saturated and unsaturated zone. Neutron probe measurements showed that there was no increase in the water content beneath a depth of ∼ 1m below the surface. Even with the exceptionally high rainfall in February 1988, neutron measurements indicated that very little soil matrix flow occurred, which implied that most of the recharge occurred along preferred pathways. A triangular finite element network was used to determine the relative saturated volume fluctuations (SVF) of the aquifers from the observed water-level changes over a period of time, which allowed determination of the storativity S and the recharge. The main advantage of the method lies in determining recharge and storativity which are both unknowns in the water balance equation and which both contribute to the water-level response. The SVF method showed that the recharge in the Karoo formations of South Africa varies between 2 and ∼5% of the annual rainfall. In areas which are overlain by a thick soil cover, the recharge is less than 3%, while recharge in hilly areas with a thin soil cover may be of the order of 5%.

Hydrochemical and Hydrogeological Impact of Hydraulic Fracturing in the Karoo, South Africa

Hydraulic fracturing has become a prevalent public and regulatory issue in most countries developing shale gas. South Africa has only recently been exposed to terrestrial gas resource development and this has created unique regulatory issues which are currently being resolved. One of the key issues under debate is the protection of groundwater resources in rural areas, since most of South Africa’s rural and some inland cities are dependent on groundwater for potable water supply. A second concern is the infrastructure requirements to handle the material movement processes during the development of each wellfield and subsequent processing of waste generated on site. Regarding the waste material production, a phased approach is required which considers the initial well development activities, production and subsequent well abandonment. Each phase has a unique risk associated with it and thus would require different management options. At the current stage most of the focus is on the initial stages of well development but the long term view has been neglected to some extent. Due to the unique geological structure of the Karoo, the presence of dolerite structures, a number of risk mitigation methods might be required to succesfully develop hydraulically fractured wells. In all aspects the chemical and hydrogeological impacts related to wellfield development cannot be ignored in the Karoo aquifer system, as it may directly influence human and environmental health. This paper will present chemical perspective on the hydraulic fracturing perspective that will deal with the impact of hydraulic fracturing fluid and flowback water. Additionally, the interaction of wellfield development and hydrogeology of the Karoo area will be discussed and how it relates to future water quality issues.