Hydrodynamic modeling of San Elijo Lagoon, California

Decisions on where to concentrate management efforts need to be guided by an ability to accurately simulate and predict physical and ecological changes. Many restoration projects experience difficulties due to a lack of understanding of the ecological response and evolution of wetland systems (Goodwin et al., 2001). There are several approaches that can be taken in analyzing a system. The appropriate selection should be based on the available data, the spatial scale of the wetland, and the physical processes governing the system (Goodwin and Kamman, 2001). Predictive tools are essential for good long-term management (Goodwin et al., 2001). The objective of this thesis is to determine whether San Elijo Lagoon is a morphologically stable environment and to investigate the movement of water and sediment patterns within the estuary. This will be answered through analysis of field data and numerical modeling of the hydrodynamics of the system. A field campaign was conducted to collect a suite of hydrodynamic and sediment data in the estuary. The development of a conceptual model was further applied to a numerical model. The Danish Hydraulic Institute’s (DHI) Mike21 software package was used to develop a two-dimensional flexible mesh hydrodynamic model. This is a depth-averaged finite volume commercial program. The hydrodynamic model was calibrated with the data collected in May, and then verified with observed conditions from July and August. The lagoon has a net depositional environment. The inlet of the lagoon is unstable due to the enforced unnatural location and meandering morphology of the inlet channel; the force of the tide is not large enough to keep the inlet clear. MIKE21-FlexibleMesh model simulations confirm that San Elijo Lagoon’s hydrodynamics is dominated by tidal forcing and freshwater inflow. The freshwater inflow, as well as the morphology of the lagoon causes an attenuation of the tidal signal. In the coincidence of extremely low tides and extremely high runoff in the watersheds feeding the lagoon the freshwater inflow at the man-made dike can have a significant impact, but only for a short period of time

Tertiary Recovery of Oil by Up-Dip Gas Injection.

A tertiary gas injection process denotes the mobilization of waterflood residual oil by gas invasion; whereas, secondary gas injection denotes the displacement of a continuous oil phase by gas. The term up-dip gas injection implies that the gas is injected in the crest of the structure in order to displace fluid down-dip where it can be recovered in a production well; whereas, an attic oil recovery process usually implies a cyclic injection/recovery process using a single well. The objective of this study is to develop a technique for modelling relative permeability in a tertiary gas injection process occurring in a high permeability, strongly water-wet sandstone reservoir for which gravity effects may be significant. The relative permeability modelling technique was developed based on the following: (1) published two-phase relative permeability data measured by conventional techniques, (2) published oil relative permeability data derived from in-situ saturation measurements, and (3) agreement between the predictions of a linear numerical simulator and the production observed during a previous laboratory study at LSU. The numerical modelling study indicated that the production behavior observed during the laboratory study can be accurately predicted using a simple power-law relationship between relative permeability and saturation. By incorporating the new relative permeability modelling procedure into a simple streamtube model, a simplified approach for screening tertiary gas injection candidates was developed. To illustrate its application, this simplified approach was applied to one of the fault blocks involved in the West Hackberry Tertiary Project. Based on the predictions of the screening model, the degree of stratification exhibited by a reservoir plays a major role in the efficiency of a tertiary gas injection process

Near Wellbore Hydraulic Fracture Propagation from Perforations in Tight Rocks: The Roles of Fracturing Fluid Viscosity and Injection Rate

Hydraulic fracture initiation and near wellbore propagation is governed by complex failure mechanisms, especially in cased perforated wellbores. Various parameters affect such mechanisms, including fracturing fluid viscosity and injection rate. In this study, three different fracturing fluids with viscosities ranging from 20 to 600 Pa.s were used to investigate the effects of varying fracturing fluid viscosities and fluid injection rates on the fracturing mechanisms. Hydraulic fracturing tests were conducted in cased perforated boreholes made in tight 150mm synthetic cubic samples. A true tri-axial stress cell was used to simulate real far field stress conditions. In addition, dimensional analyses were performed to correspond the results of lab experiments to field-scale operations. The results indicated that by increasing the fracturing fluid viscosity and injection rate, the fracturing energy increased, and consequently, higher fracturing pressures were observed. However, when the fracturing energy was transferred to a borehole at a faster rate, the fracture initiation angle also increased. This resulted in more curved fracture planes. Accordingly, a new parameter, called fracturing power, was introduced to relate fracture geometry to fluid viscosity and injection rate. Furthermore, it was observed that the presence of casing in the wellbore impacted the stress distribution around the casing in such a way that the fracture propagation deviated from the wellbore vicinity

BOAST II for the IBM 3090 and RISC 6000

BOAST II simulates isothermal, darcy flow in three dimensions. It assumes that reservoir liquids can be described in three fluid phases (oil, gas, and water) of constant composition, with physical properties that depend on pressure, only. These reservoir fluid approximations are acceptable for a large percentage of the world's oil and gas reservoirs. Consequently, BOAST II has a wide range of applicability. BOAST II can simulate oil and/or gas recovery by fluid expansion, displacement, gravity drainage, and capillary imbibition mechanisms. Typical field production problems that BOAST II can handle include primary depletion studies, pressure maintenance by water and/or gas injection, and evaluation of secondary recovery waterflooding and displacement operations. Technically, BOAST II is a finite, implicit pressure, explicit saturation (IMPES) numerical simulator. It applies both direct and iterative solution techniques for solving systems of algebraic equations. The well model allows specification of rate or pressure constraints on well performance, and the user is free to add or to recomplete wells during the simulation. In addition, the user can define multiple rock and PVT regions and can choose from three aquifer models. BOAST II also provides flexible initialization, a bubble-point tracking scheme, automatic time-step control, and a material balance check on solution stability. The user controls output, which includes a run summary and line-printer plots of fieldwide performance

An approach for wellbore failure analysis using rock cavings and image processing

There have been interests to link different cuttings/cavings to various wellbore failure types during drilling. This concept is essential when caliper and image logs are not available. Identification of wellbore failure during drilling gives more chance of immediate actions before wireline logging program. In this paper, an approach was presented based on the image processing of ditch cuttings. This approach uses the sphericity and roundness of cuttings as input data to classify caving types and subsequently determine the dominant failure type. Likewise, common definitions of cavings were discussed initially before a new criterion is suggested. This quantitative criterion was examined by observations from caliper and acoustic image logs as well. The proposed approach and criterion were implemented on ditch cuttings taken from a well in Western Australia. Results indicate that the primary failure is shear failure (breakout) due to high levels of angular cavings. However, another failure due to the fluid invasion into pre-existing fractures was also recorded by blocky cavings

Modification of mainframe BOAST II

BOAST II is a black-oil, applied-simulation tool used routinely for performing evaluation and design work in modern petroleum reservoir engineering. Personnel from the Louisiana State University Computer Science Department worked on modifying the mainframe version of this program through the simulation of two-phase flow of slightly compressible fluids in a three-dimensional porous medium. This included the construction of a FORTRAN program that uses 3-D finite elements to approximate the governing equations. The existing finite element code was adapted so that virtually any size of element could easily be incorporated into the solution scheme. This gave increased flexibility and made it possible to utilize mesh refinement techniques. Modifications to the mainframe version also involved the development and integration of radial grid systems suitable for the investigations proposed in the project

Imaging of gas-liquid annular flows for underbalanced drilling using electrical resistance tomography

The underbalanced drilling technique, which is also known as managed-pressure drilling, is playing an important role in oil and gas sector, as it reduces common conventional drilling problems such as minimal drilling rates and formation damage, differential sticking and lost circulation. Flow regime monitoring is one of the key topics in annular multiphase flow research, particularly for underbalanced drilling technique. Prediction of the prevailing flow regime in an annulus is of particular importance in the design and installation of underbalanced drilling facilities. Especially, for establishing a suitable pressure-drop model based on the characteristics of the active flow regime. The methods of flow regime prediction (or visualisation) in an annulus that are currently in use are very limited, this is evidently due to poor accuracy or they are simply not applicable to underbalanced drilling operation in practice. Therefore, this paper presents a monitoring method, in which Electrical Resistance Tomography (ERT) is used to rapidly image the prevailing flow regime in an annulus with a metallic inner pipe. Experiments were carried out using an air–water flow loop with a test section 50 mm diameter flow pipe. The two-phase air–water flow regimes are visualised in the upward vertical annulus with a radius ratio (r/R) 0.4. This paper highlights the visualisation results of only three flow regimes, namely bubble flow, transitional bubble-slug flow and slug flow. The flow regimes are visualised through axial images stacked from 50 mm diameter-pixels of 2D tomograms reconstructed with the Conjugate Gradient Method (SCG). Gas volume fraction profiles within the annular flow channel are also illustrated. The profiles are extracted using the Modified Sensitivity coefficient Back-Projection (MSBP) method with a sensitivity matrix generated from a realstic phantom in the finite element method software. The results are compared with visual observations (e.g. photographs) of the active flow regime at the time of ERT measurements

Applying Learning Theory to the Acquisition of Academic Vocabulary

Purpose: To identify effects of variability of visual input on development of conceptual representations of academic concepts for students with normal language (NL) and language-learning disabilities (LLD). Method: Students with NL (n=38) and LLD (n=11) participated in a computer-based training for introductory biology course concepts. Participants were trained on half the concepts under a low-variability condition and half under a high-variability condition. Participants completed a post-test in which they were asked to identify and rate the accuracy of novel and trained visual representations of the concepts. We performed separate repeated measures ANOVAs to examine the accuracy of identification and ratings. Results: Participants were equally accurate on trained and novel items in the high-variability condition, but were less accurate on novel items only in the low-variability condition. The LLD group showed the same pattern as the NL group; they were just less accurate. Conclusions: Results indicated that high-variability visual input may facilitate the acquisition of academic concepts in both NL and LLD college students. Specifically, it may be beneficial for generalization to novel representations of concepts. Implicit learning methods may be harnessed by college courses to provide students with basic conceptual knowledge when entering courses or beginning new units