Soft-sensing for multilateral wells with down hole pressure and temperature and surface flow measurements

Abstract

Abstract Real-time monitoring of downhole oil, gas and water flows in wells can significantly improve the production performance of these wells when this flow rate information is used to manipulate inflow control valves. An example of this is the allocation of a gas or water cone to its entrance point in a multilateral well, allowing to close down the individual well where the gas or water cone occurs, instead of closing down the complete well. Downhole monitoring of flows can be done via direct measurement. However, downhole multiphase metering is either expensive, inaccurate, or too difficult due to the harsh conditions. An alternative is to use softsensors. Softsensors estimate downhole holdups and flow rates from (relatively) cheap and reliable conventional downhole meters, such as pressure and temperature measurements, and a dynamic multiphase flow model connecting these measurements with the quantities of interest. Soft-sensing has already been investigated before for unilateral wells in Bloemen et al. (2004) and Leskens et al. (2008). In the second of these references, the simultaneous estimation of downhole oil, water and gas flows from downhole pressure and temperature measurements is considered. It is shown there that this estimation is badly conditioned (i.e. badly observable) and, thereby, not feasible in a practical situation. Using a similar approach and focussing on gas-lift wells, in Bloemen et al. (2004) it is suggested that soft-sensing with only downhole pressure and temperature measurements should work for the case that only a liquid and gas flow are estimated. In this paper, within the same soft-sensing framework as used in the mentioned two references, solutions are sought for soft-sensing of multilateral wells, both for the two-phase (gas and liquid) and three-phase (oil, water and gas) case. For that purpose, first, the question is addressed whether the unilateral two-phase case truly can be solved using only downhole pressure and temperature measurements. If so, the multilateral two-phase case is automatically solved with the corresponding soft-sensing solution simply consisting of a collection of unilateral two-phase sensors, one for each branch. It is shown that this solution is indeed feasible. After that, the three phase case is addressed. It is shown that for this case soft-sensing of multilateral wells is not possible, even when adding surface measurements and even though, as also shown here, it is possible for the unilateral well case when adding such measurements. Introduction Motivated by the discrepancy between demand for and availability of oil and gas, and by the improvement and increased availability of downhole measurement and control equipment, the oil and gas industry has embraced the "smart wells" philosophy. The main idea of this philosophy is to improve current reservoir management by combining control practices with reservoir and well monitoring techniques to get a higher recovery from a given reservoir, on the short-term and/or on the long-term, while simultaneously fulfilling constraints that are imposed out of environmental and operational considerations. In this document the focus is on the improvement of current well monitoring practice. Well monitoring can be defined as real-time measuring or estimating well production performance parameters such as water, oil and gas flow rates. These can be delivered to an operator or a control system to allow for taking steps to improve current well production performance. In particular, the application of well monitoring can improve the production of multilateral wells by determining which branches of the well are producing which fluids. This knowledge can be used to e.g. better handle gas or water breakthrough, for example by closing down only that branch of the well that produces a cone rather than closing down the complete well.