Sidewall Samples Research Articles

Coquina depositional model, Buzios Field, Brazil

Increasingly relevant as petroleum reservoirs, the coquina deposits of the pre-salt section of the Santos and Campos Basin display a wide variety of sedimentary systems that reflect a complex storm-dominated depositional environment. This complexity resulted in the deposition of numerous sedimentary facies distributed among structural lows and highs, and their understanding is an essential aspect in order to reduce exploration risks. Therefore, the aims of this contribution are twofold: (a) to understand the depositional processes during the generation of the Itapema Formation coquina deposits in the offshore pre-salt Buzios Field, Santos Basin, and (b) to establish a depositional model and identify stacking patterns in the coquina section. The facies analysis was based on rock texture, structure, allochemical composition, taxonomic composition, and taphonomic characteristics of bivalve shells, such as fragmentation, abrasion, sorting, thickness, orientation, and articulation. We described 695 thin-sections and sidewall samples distributed among ten wells in the field area. The analysis resulted in sixteen facies grouped into eight facies associations: (1) bioclastic bars, (2) washover deposits, (3) backshore, (4) bioclastic beaches, (5) tempestites, (6) tempestites with articulated shells, (7) offshore and offshore transition deposits, and (8) offshore siliciclastic mudstones. These carbonate facies associations were deposited mainly under the influence of waves and currents in isolated ramp-shaped margined platforms with low continental input associated with active faults, characteristic of the rift phase of the basin evolution. The stacking of the facies associations indicates that the Itapema Formation coquinas were deposited in an environment that became progressively shallower towards the top contact with the overlying Barra Velha Formation. Siliciclastic mudstones mark the top of the Itapema Formation immediately below the Pre-Alagoas unconformity, indicating a significant lake deepening that drowned the coquina section. The depositional model is the first one about the Buzios Field and the first about the pre-salt fields coquinas based on field-scale information.

Development mechanism of metamorphic fractured reservoirs in the Bozhong area, Bohai Bay Basin: Implications from tectonic and magmatic hydrothermal activities

The BZ19-6 condensate gas field is the largest metamorphic gas field of the Bohai Bay Basin in China and contains typical fractured reservoirs within the inner parts of an Archean metamorphic buried hill. However, the mechanisms driving its creation and evolution are not well explored. Based on core and sidewall samples, thin sections, X-ray diffraction (XRD) analysis, U–Pb age dating, fluid inclusions, and imaging logging and seismic data, the lithologies, mineral contents and reservoir characteristics of the inner parts of a metamorphic buried hill were evaluated and then the main factors controlling the fractured reservoir were further explored in the Bozhong area. The results show that the lithology mainly includes granitic gneiss, cataclastic rocks and Mesozoic intrusions, among which the granitic gneiss is widely distributed and contains a large number of light-colored minerals. The reservoir spaces are mainly composed of tectonic fractures (TFs), cleavage fractures (CFs), introduced fractures (IFs) and dissolved pores (DPs). Numerous tectonic fractures provide the main reservoir spaces and can be divided into two groups, namely those with nearly WE- and NS-oriented strikes, which are closely related to the stress fields produced by Indosinian and Yanshan movements, respectively. IFs and DPs were developed by magmatic activities, which occurred during the Mesozoic, while magmatic hydrothermal fluid had an obvious negative impact on reservoir quality, resulting in fracture fillings with minerals such as ankerite and calcite. In this study area, reservoir spaces underwent various changes, from creation to development and from filling to dissolution. Finally, we proposed an integrated reservoir model to describe the distribution and evolution of high-quality reservoirs. This study explores the formation mechanisms of fractured reservoirs within metamorphic rocks and makes practical contributions to oil and gas exploration of buried hills.

An operator view on trends in core acquisition

Abstract Core samples from the subsurface can provide unambiguous direct information to guide operator decisions. Core may be acquired with drilling equipment (full-bore core) or by post-drill wireline methods (sidewall core). Both approaches have distinct profiles of cost, risk, sample type and value, and an operator must select the most appropriate to progress business in an informed way. The option to selectively core after drilling and perceptions of lower cost and risk might indicate that sidewall coring will always be the best approach. Recent developments to increase the size and quality of rotary sidewall samples would only add weight to this view. It is not all good news for sidewall core, however. Individual sample size and total volume delivered per run are tiny; weak rock or high overbalance pressure may cause poor recovery and biased datasets; time between drilling and logging allows mud invasion and borehole relaxation, so samples are often broken and pore fluids contaminated. Sidewall sample sets therefore leave a higher degree of uncertainty when compared to full-bore core. It is this operator's view that both approaches have a role to play in reducing subsurface uncertainty, and cost, risk and value should be carefully considered when deciding which to apply.

Examination of the Seismic Characteristics of Onshore Reservoirs for Dom Gas Production Potentials in the Niger Delta Region of Nigeria

The objective of this research work is to examine two prominent dom gas reservoirs H1000 and H4000 in onshore Niger Delta region of Nigeria for potential production activities using seismic wavelet and well-to-seismic tie process to facilitate interpretation and evaluate dom gas (hydrocarbon) bearing formation. Well log and sidewall samples analyses show that the velocity attributes, high P-wave, S-wave and Vp/Vs ratio characterize sediments with high concentration of dom gas. The wells validated the presence of dom gas with total gas in place on the field estimated at 4.017 trillion ft3, GIIP 5,228.683 bscf, CIIP 445.136 MMstb, and STOIIP 34.647 MMstb, URg 4,120.494 bscf and URc 219.208 MMstbl, 90 % of which resides in the two reservoirs. The hydrocarbon intervals are estimated at H1000 (8864 ft, TVDSS) and H4000 (9577 ft, TVDSS). The borehole information was combined with seismic data to confirm lateral continuity of the dom gas reservoirs.

Pore Volume Compressibility of Unconsolidated Sand Reservoirs: Insights Gained Using Laboratory-Created Sand Pack Analogs

Pore volume compressibility is a fundamental driver of production for unconsolidated sand reservoirs. Prediction of compressibility is desirable when direct measurements on core are not available. Many characteristics of reservoir sands change simultaneously. For this reason, the controls on compressibility are difficult to isolate and interpret. We present the results of compaction experiments using laboratory-created, unconsolidated sands. In these analog sands, we change one textural or mineralogical parameter at a time to investigate the influence of that parameter on the measured compaction properties. Initially, simple quartz grain packs of varying grain sizes were used. Subsequently, additional parameters were investigated, including grain packing, angularity, sorting, feldspar content, ductile grain content, small volumes of dispersed clay, and initial sample preconditioning at stress. Multiple samples of each type were created and tested. This allowed the testing to be halted at several intermediate stresses and the samples to be examined using 2D and 3D imaging and image analysis techniques. For monomineralic quartz sand packs, grain size is a principal control on compressibility. As mean size increases from 150 to 450 μm, peak compressibility increases from 6 to 24 microsips. The depletion stress at which peak compressibility occurs decreases from 8,000 to 2,500 psi. Increasing grain angularity also increases compressibility but with smaller effect. For 150-μm quartz sands, increasing the angularity resulted in an increase in compressibility from 6 microsips for round quartz to 10 microsips for angular quartz and decreased the depletion stress required to achieve peak compressibility from 8,000 to 7,000 psi. As sorting varies from well to moderately poorly sorted, compressibility decreases, and the curve broadens as a function of depletion stress. Adding small volumes of feldspar (or other minerals that cleave) increases the compressibility more than the change resulting from changes in grain size, illustrating the importance of framework grain composition. Adding similar volumes of ductile grains results in a similar increase in compressibility to that observed for feldspar. However, when the size of the ductile grains is larger than that of the associated quartz (e.g., locally derived rip-up clasts), the increase in compressibility is significantly larger. To validate the experimental work, we compare the results of uniaxial pore volume compressibility tests on laboratory-created sands with measurements made on subsurface samples of similar texture and mineralogy. Both the shape of the compressibility curves as well as the magnitude of the compressibility are successfully reproduced. We conclude that laboratory-created sands can provide reasonable proxies for estimating the compressibility of subsurface reservoirs when intact subsurface samples are not available for measurement (e.g., only percussion sidewall samples are acquired) as long as mineralogy and texture are known.

Basement geology and its controls on the nucleation and growth of rift faults in the northern Campos Basin, offshore Brazil

AbstractThe rift phase of the Campos Basin developed during the Early Cretaceous on a heterogeneous crust comprising structures inherited from the Brasiliano‐Pan African tectonic events, mostly generated during the Neoproterozoic‐Cambrian amalgamation of western Gondwana blocks. The main rifting episode took place from the Hauterivian to the Barremian, then was succeeded by the transition and post‐rift (SAG) phases during the Aptian. Rift faults developed as a result of a progressive rotation of extension from E‐W to NW‐SE. The role of pre‐existing intra‐basement structures on the style and evolution of the rift faults was investigated using 3D high‐resolution seismic data, borehole logs and sidewall samples. Three seismic facies (SF1, SF2 and SF3) and three types of intra‐basement structures (Surfaces, Geobodies and Internal Reflections) were identified and mapped. They represent, respectively, contrasting levels of seismic anisotropy, interpreted as metamorphic foliation, and ductile shear zones that bound rock units with particular seismic facies signatures. Sidewall cores show that banded biotite‐gneiss is the predominant rock type in the eastern half of the study area, while more homogeneous granitoid is the dominant lithology on the west. Such a binary division of lithotypes is consistent with the distribution of mapped intra‐basement seismic facies and features. The contrasting basement heterogeneity across the study area is the major control in the strain distribution during rifting. Where the basement is highly heterogeneous, the pre‐existing fabric was selectively reactivated whenever its orientation was favourable, resulting in faults forming progressively as the extension direction rotates, whilst shallower low to very low angle basement fabric were cross‐cut by rift faults. Where the basement is homogeneous, only early formed faults remain active throughout the rifting.

The challenging carbonates from the Pre-Salt reservoirs offshore Brazil: facies, palaeoenvironment and diagenesis

The Santos Basin in offshore Brazil belongs to the major carbonate reservoir province known informally as the Pre-Salt. A detailed analysis of cores, plugs, sidewall samples and thin-sections from two wells in a sector of this basin was undertaken, on an interval corresponding to the upper part of the Lower Cretaceous Barra Velha Formation. Several facies and subfacies types have been classified independently from existing terminologies for these carbonate rocks. The specific features of the studied material are discussed and comparison to other nomenclatures is made. The facies include, namely: laminated argillaceous limestones; in-situ grown carbonates with a range of singular morphologies (of debatable origin), from laminar crust, stromatolitic-like fabric to robust or delicate shrubby structures; intraclastic grainstones/packstones, whose clasts may be mostly remnants of shrubs or differ; spherulitic grainstones/packstones; locally, oncoidal grainstones/rudstones and packstones/floatstones.These rocks are interpreted as depositional and diagenetic or essentially diagenetic facies. The latter are regarded as originally similar to some of the former ones but now "disguised", because they were differently impacted by diagenesis processes over different time periods. Recognizing these effects is relevant, because they produce different reservoir facies characteristics and quality. The best potential reservoirs are some of the depositional and the mostly diagenetic facies.The palaeoenvironmental setting of the studied facies fits into a shallow, low-gradient lake. Charophyte and other possible, unidentified algae, albeit extremely rare, are documented for the first time. The facies pattern shows some cyclicity, suggestive of both water depth and chemistry changes.If the interpretations in this study are correct, they (i) reinforce existing datasets to improve understanding of these facies; (ii) raise caution concerning some facies previously considered as separate depositional products, which in fact may instead result from diagenetic modifications; (iii) stress the relevance of microfacies analysis and diagenetic petrography to help deciphering how diagenetic modifications impact on reservoir facies properties and quality; and (iv) add new issues to the naming and textural classification problems of the Brazil Pre-Salt carbonates.

Depositional and diagenetic processes in the pre-salt rift section of a Santos Basin area, SE Brazil

ABSTRACT This study aims to provide a better understanding of the genesis and diagenetic evolution of the rift pre-salt deposits of the Santos Basin, in the context of evolution of the lake system in which the sediments were deposited. The study of the syngenetic and early diagenetic constituents and their paragenetic relations allowed reconstruction of the evolution of lacustrine environmental conditions during the rift stage in the central area of the basin. A petrological study was performed on cores and sidewall samples of a well drilled through the pre-salt rift section in the Lower Cretaceous of the central Santos Basin, eastern Brazilian coast. The studied rocks consist of carbonate and non-carbonate intrabasinal components, with extrabasinal non-carbonate contribution. During Barremian early rift sedimentation, intense syngenetic precipitation of magnesian clay minerals under highly alkaline conditions deposited arenites constituted by stevensite ooids and peloids, as well as hybrid, resedimented deposits of the Piçarras Formation. Freshening of the lacustrine environment provided the conditions required for the proliferation of bivalves and sedimentation of the coquinas of the Itapema Formation. The massive structure of the deposits, poor sorting, chaotic to concave-up orientation of the bioclasts, and mixing of bivalves with stevensite particles indicate redeposition by gravity flows. The main diagenetic processes in rift stevensite and hybrid rocks are the cementation and replacement of grains by calcite, dolomite, and subordinately, silica and smectite. Siliciclastic-volcaniclastic mudrocks present intense replacement by dolomite. In the bivalve rudstones, the main diagenetic processes were dissolution of the shells and cementation of the intraparticle and interparticle pores by calcite and, subordinately, dolomite and silica.

Calcareous nannofossil assemblages of the Late Cretaceous Fiqa Formation, north Oman

Abstract. This study presents the first detailed calcareous nannofossil assemblage data from the Late Cretaceous succession of the subsurface Aruma Basin, north Oman. The taxonomic description and documentation of assemblage composition are based on extensive quantitative analysis of ditch cuttings and side-wall samples from eight hydrocarbon exploration wells across north Oman. The samples studied from those wells cover the Coniacian to earliest Maastrichtian deep marine shales and marls of the subsurface Fiqa Formation. These fine-grained siliciclastic deposits often yield moderately to well-preserved nannofossil assemblages, especially in the Campanian intervals. Consequently, diverse assemblages have been recorded from the Fiqa Formation, with a total diversity of ∼200 species, including two new species, Staurolithites ormae sp. nov. and Chiastozygus fahudensis sp. nov., which are illustrated and described. Extensive imaging of this diversity is provided here, as are stratigraphic distributions of the main components from a key reference well in north Oman, W-4. Poorly described groups such as Staurolithites are closely investigated and their utility for stratigraphic applications is highlighted. Relative abundances of nannofossil taxa with strong palaeoenvironmental preferences have revealed new insights into the palaeo-productivity of the basin. High-fertility species like Discorhabdus ignotus, Biscutum constans and Zeugrhabdotus erectus show substantial variations in abundance throughout the Fiqa Formation, reflecting long-term shifts in the productivity conditions of the basin. This is supported by abundance patterns of Watznaueria barnesiae and Micula staurophora that show a broadly inverse correlation with the high-fertility species. The Fiqa Formation represents a key regional seal unit for the Cretaceous hydrocarbon reservoirs of Oman, as well as being a productive unit elsewhere in the Arabian Peninsula. Beyond the Aruma Basin of Oman, this study will provide a key reference point for future biostratigraphic or palaeoenvironmental analyses of the Late Cretaceous calcareous nannofossil assemblages across the Middle East and other southern Tethyan areas.

Poisson's ratio prediction through dual stimulated fuzzy logic by ACE and GA-PS

Poisson's ratio is one of the most important rock mechanical parameters having significance in both planning and post analysis of wellbore operations. Laboratory measurement of this parameter covers a broad range of costs, including sidewall sampling, preservation, and laboratory tests. This study proposes an improved strategy, called dual stimulated fuzzy logic by ACE and GA-PS for determining Poisson's ratio from conventional well log data in a rapid, precise, and cost-effective way. Firstly, conventional well log data are transformed to a higher correlated data space with Poisson's ratio through the use of alternative condition expectation (ACE) algorithm. This step simplifies the convoluted space of the problem and makes it easier to solve for fuzzy logic. Subsequently, transformed conventional well log data are fed to fuzzy logic model. To ensure that optimal fuzzy model is constructed, a hybrid genetic algorithm-pattern search (GA-PS) technique is employed for extracting fuzzy clusters (or rules). This step sets fuzzy logic to its optimal performance. The propounded strategy was successfully applied to data from carbonate reservoir rocks of an Iranian Oil Field. A comparison between present model and previous models showed superiority of current study. • Dual stimulated fuzzy logic was used for Poisson's ratio prediction. • First, problem space is simplified by ACE for fuzzy logic. • Then, structure of fuzzy model is optimized by GA. • Integration of fuzzy logic, ACE, and GA develops an improved model. • This study surpasses previous methods in terms of R -square and MSE.

Electrofacies Modelling and Lithological Classification of Coals and Mud-bearing Fine-grained Siliciclastic Rocks Based on Neural Networks

The identification of lithofacies from well is usually an interpretative process based on geophysical logs since core and sidewall samples are not usually available. Despite being always sampled and described, cuttings are useful only as a reference for determining the rocks because a number of problems occur during the drilling and sampling activities. Well logs are in situ continuous records of different physical properties of the drilled rocks, which can be associated with different lithofacies by experienced log analysts. This task needs a relatively great amount of time and it is likely to be imperfect because the human analysis is subjective. Thus, any alternative method of classification with high accuracy and promptness is very welcome by the log analysts. This paper is based on Neural Networks (NNs) applied in well data from the Leão Coal Mine, southern Brazil, in order to classify organic mudrocks, coals and siliciclastic sandstones, the main rocks present in the Rio Bonito and Palermo formations, by using their well logs as database. The training and validation set of the NN contain data from eight cored and logged boreholes. The input included 409 values of depth and logs of gamma-ray, spontaneous potential, resistance and resistivity for each electrofacies. The neural network model was the feedforward multilayer perceptron (MLP) and the neural networks were trained with variations of the backpropagation algorithm: Levenberg-Marquardt and Resilient backpropagation. Although an accuracy of approximately 80% had been achieved in the general classification, discrepant accuracies in the classification of the different electrofacies are discussed in order to better understand the reasons that affected negatively the NN performance.

Reservoir fluid differentiation case study from Oredo field in the Niger Delta Nigeria

The formations in the Niger Delta-Nigeria consist of sands and shales with the former ranging from fluvial (channel) to fluviomarine (Barrier Bar), while the latter are generally fluviomarine or lagoonal. These Formations are mostly unconsolidated and it is often not feasible to take core samples or make drill stem tests. Formation evaluation is consequently based mostly on logs, with the help of sidewall samples and wireline formation tests. From the comparison between the FDC/CNL (Formation Density/Neutron Logs spell out) separation and SWS (sidewell sample) petrophysical data, it is possible for fluid differentiation to be done from SWS measurement with a good measure of reliability, in the absence of production data and FDC/CNL logs. This may not apply to liquid – rich gas and condensate reservoirs. The FDC/CNL log responses are better defined in thick clean sands as against thin marginal sands in this case study. Key words: Formations, Gas/Oil, Sands and Shales, Hydrocarbon, Porosity, Resistivity.

Calcareous nannofossils and the Santonian/Campanian and Campanian/Maastrichtian boundaries on the Brazilian Continental Margin: historical overview and state of the art

Recognition of the Campanian stage on the Brazilian Continental Margin, using calcareous nannofossils, has been historically problematic. This paper constitutes an overview of earlier works, showing how nannofossil biostratigraphic ideas have evolved since Troelsen & Quadros provided the first biozonation of this region in 1971. Recent studied have provided data which have helped to clarify these apparent biostratigraphic problems, and allows this region to be placed in a global biostratigraphic context. The earliest researchers identified the Santonian/Campanian boundary by the last occurrences of ‘ Lithastrinus grillii’ and Petrobrasiella venata. P. venatawas later abandoned as an index species due to its rarity and, instead, the last occurrences of Marthasterites furcatusand ‘ Lithastrinus grillii’ became the most-used markers. However, the stratigraphic age of these biohorizons diverged from those quoted in the literature. In the Brazilian basins, these extinctions, rather than having occurred in the Campanian as was recorded elsewhere, were considered to mark the top of the Santonian, as suggested by correlations with other microfossil groups (primarily foraminifera and palynomorphs). To explain this phenomenon, the existence of a condensed sequence was postulated for most of the Brazilian marginal basins, where the uppermost Santonian deposits were apparently indistinguishable from those of the lowermost Campanian. In line with current correlations presented in the nannofossil literature, and with new information obtained from core and side-wall samples, it is now believed that the extinction of these species didoccur in the Campanian in the Brazilian basins, whilst the last occurrence of Lithastrinus moratus(previously misidentified as Lithastrinus grillii) has become a useful Santonian marker. Thus the Santonian/Campanian boundary in Brazil lies in a stratigraphic position similar to elsewhere in nannofossil terms, that is below CC18. The Campanian/Maastrichtian boundary was initially characterised in nannofossil terms in Brazil by the last occurrence of Broinsonia parca constricta, and later by the last occurrence of Eiffellithus eximius. Recently acquired data has shown that the sequence of events in the Brazilian marginal basins is similar to that of the Sissingh/Perch-Nielsen standard biozonation scheme through this interval. Again, correlations in the literature with the recently defined boundary (in macrofossil terms) thus allow the boundary to be determined between the last occurrences of Broinsonia parca constrictaand Uniplanarius trifidus, that is, in CC23b.

Predicting Oil Quality from Sidewall Cores Using PFID, TEC, and NIR Analytical Techniques in Sandstone Reservoirs, Offshore Cameroon: ABSTRACT

Cameroon reservoirs contain oil and gas that have migrated vertically from deeper buried thermally mature marine shales. Several shallow reservoirs also contain biogenic gas. Generally, lower gravity oils found in the shallow reservoirs have undergone various degrees of biodegradation. Deeper accumulations are higher gravity {open_quote}primary{close_quote} oils. The biodegraded oils are characterized by lower gravities, higher acid numbers, higher sulfur contents, and higher viscosities than their non-biodegraded counterparts. Oil quality (API gravity and acid number) has a significant impact on the development economics. It is important to obtain as much geochemical information as possible from the limited volume of oil contained in conventional sidewall samples because borehole conditions often preclude the possibility of running a wireline test tool (the MDT) to obtain a fluid sample. An analytical program of PFID (Pyrolysis Flame Ionization Detection), TEC (Thermal Extraction Chromatography) and NIR (Near Infra-Red spectroscopy) was conducted on a set of {open_quote}calibration{close_quote} oils and the data were used to develop both empirical and quantitative predictive criteria for estimating crude oil properties of gravity, acid number, sulfur content, and viscosity. Sidewall samples provide sufficient material for these micro-analytical techniques. The individual sidewalls were split for geochemical analysis with the remaining material available for petrographicmore » analysis. The PFID and TEC techniques were run on the {open_quote}rock{close_quote} sample containing the oil. For the NIR technique, the oil was extracted from the sample with an organic solvent and the extract evaluated. Results will be presented for sidewall core samples obtained from four oil sands encountered during a two well exploratory program.« less

Subsurface radionuclide investigation of a nuclear test

This paper reports on an environmental investigation into the vertical distribution of radionuclides from a nuclear test. Dalhart is the name of an underground nuclear test that was executed at the Nevada Test Site at a depth of 2100 ft on October 13, 1988. The test occurred below the static water level of 1667 ft and created multiple radioactive isotopes or fission products. These radioactive isotopes penetrated the surrounding formations and chimney region above the test and were retained there. A 19° 9- 7 8 -inch diameter slant hole was drilled to sample the geologic material in the chimney region above the Dalhart test for the purpose of assessing the distribution of radioactivity in and around the shot site. A 30-ft core recovered from a vertical depth of 1628 ft in the collapsed zone or chimney region and above the original static water level was found to be free of radionuclides. Drilling was completed to a vertical depth of 2156 ft with the present static water level at a vertical depth of 1644 ft. Gamma-ray spectroscopy log measurements, made within the drill pipe while drilling fluid was pumped through this pipe, indicate that radioactive material produced by the test was present from the vertical depth interval of 1746–2156 ft. Side-wall samples acquired from the vertical depth interval of 1721–2089 ft and analyzed in the field contained radionuclides such as 137Cs, 125Sb, 106Ru, plus the natural radioactive background of potassium, uranium, and thorium. These samples were sent to Los Alamos to determine the complete radionuclide content at each depth. These analyses were used with the gamma-ray spectroscopy logging data to determine the subsurface vertical radionuclide distribution at the Dalhart site.

A Novel Procedure to Increase Well Response to Matrix Acidising Treatments

ABSTRACT Matrix acid treatment design in clastic formations has traditionally been based on field experience or core-flushing tests. The choice of stimulation fluid compositions has frequently been limited to full-strength (12% HCl/3% HF) or half-strength (6% HCl 1.5% HF) mud acid. These practical rules of thumb have given good results but have not always ensured an optimum acid/rock reaction, i.e. the maximum net dissolution of the rock minerals and impairment coupled with a minimum of secondary precipitate formation. This paper shows how mathematical modelling of the chemical equilibria using a thermodynamic simulator can identify the ideal acid formulation, even when cores are not available. The input for the simulator is the mineralogical composition of the formation (from cuttings, sidewall samples or cores), the nature of the inorganic damage and the rock's porosity-permeability relationship. Chemical equilibria are calculated for different acid compositions taking into account the dynamic nature of the acidising process: the drop in acid strength during reaction with the damaged formation, and the changing rock composition due to attack by the acid. The results are translated via the porosity-permeability relationships into a theoretical well-productivity improvement factor. The acid composition which gives the highest productivity improvement is the optimum or Specially Formulated Mud Acid (SFMA) composition. It is important to note that the SFMA concept is not limited to HCl or HCl/HF acid formulations, but can also deal with other types such as Buffer Regulated HydroFluoric Acid (BRHFA). It will be shown that different rock compositions lead to very different SFMA formulations. SFMA recipes have been tested in core-flushing experiments, and have been found to give better performance than standard mud-acid recipes. The procedure was further validated by analysing the spent fluids regained by back production after field treatment. Field case histories in which improved production results were achieved using the SFMA approach are described.

Significance of Lignites in Paleocene Middle Wilcox Stratigraphy in East Central Louisiana

ABSTRACT Lignites are a ubiquitous component of the stratigraphic sequence in the deltaic depositional environment of the Wilcox Group. In the Middle Wilcox, between the Big Shale and the upper Minter oil sands, 11 principal lignites, ranging in thickness from a few inches to 5-10 feet, have been observed on E-logs and corroborated by conventional cores and side wall samples. These extensively correlative lignites commonly overlie distributary channels and overbank bay fills. The lignites are usually overlain by shales indicating that compactional subsidense or sea level rise returned the area to shallow marine conditions. Assuming a lower delta-plain setting, the upper lignite surfaces commonly represent marine-flooding surfaces or parasequence boundaries. Peat accumulation resulting in lignites appears to represent only minor hiatuses between aggradational sands and interdistributary bay marine deposits. Productive Minter oil sands (2 ft - 12 ft) are closely associated with lignites and may be bounded above and below by them (Bee Brake and North Bee Brake Fields). These lignites possibly act as oil migration conduits. Organic petrography and geochemistry indicate that they are good potential source rocks for hydrocarbons. The association of these lignites with possible methane gas production is as yet an uninvestigated possibility in east central Louisiana.

Niger Delta reservoir geology: historical growth of the sedimentological model and its application to field development

Abstract In 1953 when the exploration in Nigeria moved towards the Niger Delta, it was realised that the recent delta could be used as a guide to the past. Consequently many studies were carried out between 1953 and 1964, ranging from surveys of the shallow offshore, the coastal zone and the rivers to an extensive shallow coring campaign (1960–62). The studies of the recent delta were soon augmented by the analysis of cores from several newly discovered fields (1963–69) and also by the description and grain size analysis of numerous side-wall samples. Around 1970 the basic facies types comprising the individual deltaic cycles of the hc-bearing paralic sequence were well established. The facies types occur in a very repetitive pattern of combinations and successions which facilitates recognition and correlation. A fairly reliable system of diagnostic criteria was worked out based on petrophysical logs and side-wall samples and calibrated by the cores and the recent delta. With the sequencial introduction of new logging tools the diagnostic methods are being refined while additional cored wells continuously enlarge the data base. Detailed seismic analysis at reservoir level is showing considerable promise for sand body delineation. For each facies a representative data base of sand body geometry could be compiled based on detailed reservoir correlations and recent examples. There is a good understanding of typical permeability and grain-size distributions. Knowledge on shale intercalation continuity is particularly useful in the planning of well completions to retard gas or water breakthrough to the well bore. It was shown that permeability can be computed from log derived porosity and grain-size distribution measured on side-wall samples. The practical value of the reservoir geological research lies in the possibility to correlate and model reservoirs in a reliable and consistent manner. Reservoir and well behaviour can be predicted and where necessary reservoir simulation models can be constructed. Continuous updating of reservoir studies has demonstrated the validity of this approach by checking past predictions and analysis of infill wells. In this way the production planning and reserve estimation is given a solid base.

Fermentation characteristics, digestibility and performance of ensiled swine waste, wheat straw and cane molasses fed to sheep

In four full-scale bunker silos were ensiled 16·5 tons of different mixtures of swine waste, wheat straw and cane molasses. One mixture had 22% swine waste, and the other one 44% (DMB). Swine waste was collected from an open concrete growing-finishing swine feedlot. During the first six weeks samples were taken from the silos and analyzed for pH, lactic acid, total coliforms, clostridia and aerobic and lactic acid bacteria. After three months, silos were opened and evaluated for physico-chemical, microbiological and nutritional characteristics. Two performance trials on sheep were conducted, one to determine the nutritive value of the silages with 22 and 44% of swine waste and the other one to evaluate the inclusion of the silage with 44% swine waste at 20, 40, 60 and 80% (DMB) in sheep diets. Dry matter, crude protein, acid detergent fiber and neutral detergent fiber digestibilities were determined for silages with 22 and 44% swine waste in eight sheep. Evaluation of storage stability in samples from different levels in bunker silos indicated low stability in surface, bottom and side wall samples and fairly good stability in samples from 50 cm depth. Those samples had a pleasant aroma and appearance similar to that of a good quality haylage. Typical manure aroma was not present in waste silages. Twenty-two percent swine waste silage reached an average pH 4·4 while 44% swine waste silage reached an average pH4·6. After 7 days of fermentation, total coliforms were completely destroyed. Clostridia and aerobic bacteria tended to be decreased by ensiling. Average daily gain and digestibility of crude protein were higher for sheep fed 44% swine waste silage than sheep fed 22% swine waste silage, but lower than sheep fed basal diet ( P < 0·01). Feed efficiency was different for the three diets ( P < 0·01). Dry matter intake was similar for sheep fed 44 and 22% swine waste silage ( P > 0·01) but lower than sheep fed basal diet ( P < 0·01). Average daily gain and dry matter intake decreased with increasing levels of 44% swine waste silage ( P < 0·01). Feed efficiency was not different between the four treatments ( P < 0·01).

Examination of encrustations in an unlined borehole using the sidewall sampling technique : Howsam, P; Brassington, F C; Lucey, P A Q J Engng GeolV22, N2, 1989, P139–144

Examination of encrustations in an unlined borehole using the sidewall sampling technique : Howsam, P; Brassington, F C; Lucey, P A Q J Engng GeolV22, N2, 1989, P139–144