Wellbore Flow Simulations For Texas-Louisiana Geopressured Reservoirs
Authors: Lamb J. Parker , Rhode David L. 
Published: December 2007

Computed variations of wellhead fluid energy levels with elapsed production time are presented as functions of physical parameters such as untapped reservoir pressure and temperature, flow tube diameter, and flow tube scale growth rate. For a typical reservoir the wellhead delivery system performance is found to be generally linear in character. Thus the performance of similar reservoirs is easily obtained by interpolation and extrapolation of the present results.

Introduction

Increased energy demands in the presence of ever increasing uncertainties regarding future supplies have greatly expanded the horizon of our nation's economical energy sources. Geopressured geothermal energy may be capable of providing one component of our total future energy requirements. For example, Dorfman and Kehle have estimated that the brine energy content within such geopressured aquifers in Texas alone may be as high as 20,000 MW centuries; moreover, any coexisting natural gas would contribute considerably more to the total resource.

Among the most promising geopressured aquifers along Gulf coastal fairways are those in Brazoria County, Texas. These reservoirs exist as stratified layers at depths ranging from 13,500 to 16,500 ft (approx. 4000 to 5000 m); they exhibit virgin pressures between 10,000 and 12,500 psia (approx. pressures between 10,000 and 12,500 psia (approx. 69,000 to 86,000 kPa) and temperatures between 300 deg. F and 350 deg. F (149 to 177 deg. C). Moreover, the geopressured brines could contain in solution as much as 45 cu ft/bbl (.34 kmol/m) natural gas.

Unlike a conventional power source such as a steam boiler, which can deliver a high enthalpy working fluid to an energy conversion system at a uniform rate over many years, a geopressured reservoir-wellbore system represents an energy source whose output is transient due to reservoir depletion and scale growth in the flow tube. During wellbore ascension, a considerable amount of mechanical energy (which is manifested as pressure) is consumed in overcoming the gravitational force and a lesser quantity is converted into thermal energy by frictional effects while a portion of the thermal energy is conducted into the geological formations. As the brine mixture ascends the wellbore its pressure and temperature decrease and hence gas solubility decreases. This process results in the continuous nucleation of dissolved gas into small vapor bubbles.

The liquid portion of the two-phase wellhead fluid contains both mechanical and thermal energy which must be utilized efficiently by the surface conversion plant. Simultaneously, the gaseous phase must be efficiently separated prior to phase must be efficiently separated prior to utilization. Preceding the design of surface plants, however, must be a thorough understanding of the reservoir-wellbore delivery system so that gross levels of energy available to a surface plant can be estimated. Thus, the present paper will review a simulation technique for predicting the variation of wellhead available energy with elapsed production time, and discuss the use of these variations production time, and discuss the use of these variations to obtain rapid estimates for preliminary optimization of candidate systems.

Specific predictions are presented for a model reservoir based on extensive geological analysis of Texas Gulf coast fairways. The impact of such parameters as initial reservoir temperature and parameters as initial reservoir temperature and pressure, flow tube diameter, amount of dissolved pressure, flow tube diameter, amount of dissolved gas recovered, and mineral scale growth rate are presented and discussed. Important design presented and discussed. Important design compromises are also highlighted.

WELLBORE FLOW MODEL

For the present discussion, the wellbore flow of geopressured geothermal fluid is simulated with a one-dimensional homogeneous two-phase model for liquid and vapor. The liquid phase is characterized by a solution of water and methane, while the vapor phase is considered to be pure methane.