Ekwere J. Peters

A model for predicting the density of oil-based muds at high pressures and temperatures

A compositional material-balance model was used to predict the densities of diesel- and mineral-oil-based muds at elevated pressures and temperatures. The authors measured the densities of diesel oil, two mineral oils, and calcium chloride solutions from 78 to 350{degrees}F and from 0 to 15,000 psig. The measured densities were used in an existing compositional material-balance model to predict the densities of 11- and 17-lbm/gal oil-based muds. The authors also measured the densities of these muds at elevated pressures and temperatures and compared them with the predicted values. The results show excellent agreement between measured and predicted densities. The experimental density data were used to predict downhole densities and static wellbore pressures for the oil-based muds. Results show that the mineral-oil muds are not only more compressible than the diesel-oil muds, but also more susceptible to thermal expansion. Therefore, all the oil-based muds tend to give essentially the same static-wellbore-pressure profile.

The Effect of Instability on Relative Permeability Curves Obtained by the Dynamic-Displacement Method

A study was undertaken to investigate how instability would affect the oil/water relative permeability curves obtained by the dynamic-displacement method. In this method, stable Buckey-Leverett displacement theory is used to calculate relative permeability curves from coreflood data. Thus, to obtain the true relative permeability curves by the dynamic-displacement method, the coreflood must be stable. However, the method frequently has been applied to unstable corefloods. The consequence of this application of the method has not been previously reported. The authors compared oil/water relative permeability curves from steady-state and dynamic-displacement experiments at several levels of instability. The results showed that the dynamic-displacement relative permeability curves deviated significantly from the steady-state curves as the degree of instability increased. This observation indicates the need to scale laboratory relative permeability measurements to account for instability. To obtain representative relative permeability curves for numerical modeling of a reservoir, laboratory displacement experiments should be conducted at the same degree of instability as that in the reservoir.

A Stability Theory for Miscible Displacement

This paper presents a dimensionless stability number and its critical value for predicting the onset of instability in miscible displacements. Also presented is a novel technique for determining the dispersion parameter required for calculating the stability number. The technique combines spectral analysis with computer image processing of the solvent fingering pattern.

An experimental study of casing performance under thermal cycling conditions

The authors have investigated the behavior of casing pipe body and connections under simulated thermal recovery conditions. The study, performed in a new computer-controlled thermal-well simulator, examined the thermal stress behavior and leak resistance of pipe and connections at temperatures up to 354{degrees}C (670{degrees}F) under severe loading conditions similar to those encountered in thermal wells. We also studied the biaxial collapse resistance of the casing under the large axial tension that would exist after the cooling period in a steam-stimulation process. Results indicate that in a steam-injection process in which the casing is maintained at its maximum temperature for a period of time, stress relaxation can occur in a constrained production casing, resulting in the development of excessive tensile stress during the cooling period. Premium connections with metal-to-metal seals maintained gas leak resistance at temperatures up to 354{degrees}C (670{degrees}F), whereas the connections without metal seals did not. The biaxial collapse resistance of casing under large axial tension depended on the stress/strain characteristics of the material.

Computer image processing: A new tool for studying viscous fingering in corefloods

Computer image processing technology was used to study the pattern of viscous fingering in laboratory corefloods. The fingering patterns in unstable immiscible corefloods were captured and photographed with the aid of a fluorescent dye. The photographs were digitized and stored as computer image files for subsequent processing. A program was written to count the fingers, to determine their sizes, to compute the sweep efficiencies at the core cross sections, and to determine the frequency contents of the fingering patterns. The frequency contents were used in conjunction with stability theory to estimate the effective interfacial tension (IFT) required for calculating the stability numbers of the floods. Results show that computer image processing technology can be used to study the performance of laboratory corefloods quantitatively.

A Microcomputer-Based Imaging System for the Visualization of Fluid Displacements

Presentation of a PC-based imaging workstation that uses a video camera and appropriate computer hardware and software to digitize, display, and store images of fluid flow patterns in real time. The system includes image-processing software so that the digital images can be quantitatively analyzed

Influence of Compound Grease on the Performance of Premium Connections

The authors investigated the harmful effect of compound grease on premium connections. The pressure buildup of compound grease trapped between threads was measured during and after makeup of the connection. The resulting hoop stress in the coupling and the reduction in the seal interference were also measured. Mathematical models were developed to calculate coupling stress and grease pressure. Results indicate that for the conventional API buttress-thread profile, the pressure of trapped compound grease remains permanently high after the connection is made up because of the small thread-clearance area. This high pressure results in a high coupling stress and a large reduction in seal interference diameter, thereby degrading the integrity of the connection. To solve this problem, the thread profile was redesigned with a thread-clearance area larger than that of conventional buttress thread. The connection with the new thread design showed a significantly lower pressure buildup, coupling stress, and seal interference reduction than that with the conventional thread design. Therefore, premium connections with the new thread design are recommended over those with conventional buttress thread design for wells in hostile environments.

Simulation of unstable miscible displacements by finite element method

A modified finite element method was developed to simulate unstable miscible displacements in heterogeneous porous media. The modification was to evaluate the transport properties at the center of the elements, instead of at the numerous gauss points. This averaging procedure reduced the computational time by as much as thirty percent compared to the standard finite element method while still retaining the fingering behavior of the unstable displacements. Further, the modified method showed less grid orientation effect and resulted in a more robust numerical model than the standard finite element method.

Expected Returns on Investment in the Giddings, Austin Chalk (Gas) Field

An economic evaluation has been performed to determine the expected returns on investment for a drilling program in the Giddings, Austin Chalk Gas Field. Using projected production schedules of selected well types from the encountered reserve distribution, computer based cashflow analyses were performed under six hypothesized future scenarios of varying hydrocarbon pricing, demand, taxation, and inflation. For the six future scenarios studied, the Giddings Gas Field drilling program was found to produce a positive, after tax net present value, using 15% as a discount factor. Furthermore, the returns on investment were higher for an independent than for a major producer. Results suggest a drilling program initiated in the near future will prove to be an attractive economic venture.