3 research outputs found
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Rapid Casing Corrosion in High Temperature Liquid Dominated Geothermal Fields
Downhole logging and workover operations on 12-20 year old wells in several high temperature, liquid-dominated geothermal fields in New Zealand has shown that severe corrosion has commonly occurred in the production casing string where this is unprotected by larger diameter casings. To date corrosion products from only one well have been examined in detail. These indicate that corrosion attack commences at the outer casing wall and continues at a rate as great as 0.8mm/year. Rapid corrosion has been attributed to neutral or slightly acid high bicarbonate waters formed by the absorption of steam and gas into shallow aquifers not directly connected to the deeper, high chloride reservoir
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Modelling from a User's Point-of-View
In the context of this paper the 'user' includes all those involved in the management of a geothermal resource. The objective of reservoir modelling and simulation, in my view, should be to supply these 'users' with appropriate and reliable data so that they can make sound decisions, whether these be in financial or engineering fields. The scale of models would thus extend from a single w e l l , which may require a workover to remove mineral deposition costing, say, 100 million. Geothermal development work can involve high risks. With the aid of modelling it should be possible to quantify risks, and, if these are considered excessive, to identify what data should be obtained to reduce them. Schemes relying on a single well are particularly hazardous. In 1974 an alfalfa drying plant was built at Broadlands next to well BR7. This well had previously been flow tested for three years, showing very reliable discharge characteristics. But when opened to supply the drying plant, the well did not perform satisfactorily and a 350 m pipeline was built to supply steam from another nearby productive w e l l . A l l geothermal developments involve drilling and this, also, is a risky business. In New Zealand, over the last six years, the time to drill a 1200- 1400 m investigation well shows variations of 240% about the average (using the same rig and equipment). The time for workovers is even more variable, due to unforeseen downhole conditions affecting progress - an overrun of 100% would not be unexpected. What information does the user want? Basic data w i l l be the power-life and likely discharge characteristics of a field given certain operating conditions. For example, i f we have a liquid dominated field, the design engineers w i l l require information about changing enthalpy, gas and total flow for the system as a whole and for each individual well. There is no point in fitting a well with equipment to handle 100 t/h of separated steam if, after three years' production, it will only produce 50 t/h. Those providing finance for a scheme will require an assurance of its viability. How many years can the field sustain the projected withdrawal rate? For a well repair operation, those providing funds will want to know the chances of success and how much steam the well might produce if the operation succeeds. These factors must be balanced against the estimated cost of the operation, which itself identified in the first place. In a field which has been operating for some years, the question arises: What do we do now? Do we increase production, or do nothing, or maybe manage the field by operating only selected wells. Whatever the choice, predictions of future field and well performance are required to allow the field operator to evaluate the economics of alternative schemes
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An Improved Algorithm for Spinner Profile Analysis
A new approach to the analysis of spinner data is proposed, where the complete set of data (depth, tool velocity, frequency) is regarded as a single set of observations to be fitted to a model of fluid structure (depth, fluid velocity) and a model of tool performance. This approach of treating the entire data set on the same basis is considered to be better than previous methods and more consistent mathematical practice. Multiple observations at calibration stations count in proportion to their number, but repeat observations at other depths contribute equally to the tool model or calibration. If measurements only at a fixed station are considered, the approach is identical to standard calibration cross-plotting. This analysis method generates from a set of measurements at different tool velocities, a single best estimate of fluid velocity against depth. This can be subjectively interpreted. Such interpretations are free of tool bias. If multiple flow rates have been used, a velocity profile is generated for each flow rate. Comparison between these velocity profiles can be used to generate a model of well diameter and feed zone structure