Single Well Chemical Tracer Tests Research Articles

Low-Salinity Waterflooding for EOR in Field A of Western Offshore Basin: A Pilot Study Analysis with Laboratory and Simulation Studies—Early Observations

Carbonate reservoirs hold vast oil reserves, but their complex properties make traditional enhanced oil recovery (EOR) methods challenging. This study explores the application of low-salinity water flooding (LSWF) as a novel EOR method for India’s largest offshore carbonate oil field. Conventional EOR techniques were deemed unsuitable due to reservoir heterogeneity, pressure decline, high temperature, and the offshore location. Favorable factors for LSWF included successful seawater flooding history, medium-weight crude oil, and existing infrastructure. Following core flooding experiments demonstrating a 6–16% increase in oil recovery, a multi-pronged evaluation process was implemented. Single-well chemical tracer tests (SWCTT) and reservoir simulations confirmed the potential of LSWF. A specific target area was chosen based on reservoir characteristics, production data, and available facilities. Simulations predicted a 1.5% incremental oil recovery using diluted seawater (25% salinity) at 30% pore volume injection. After a positive techno-economic analysis, the first offshore LSWF project in India was completed within 3 years. Initial monitoring results are encouraging. This study highlights the successful journey of LSWF from concept to field deployment in a challenging carbonate reservoir, showcasing its potential for revitalizing such fields. Furthermore, this work provides valuable data relevant to Indian offshore environments, where factors like salinity, mineralogy, and crude oil composition pose unique challenges compared to other LSWF applications. These detailed data fill a critical gap in the existing literature.

Review of Offshore Chemical Flooding Field Applications and Key Lessons Learned

Summary This paper presents an overview of both current advancements and field applications of offshore chemical flooding technologies. Along with offshore oilfield development strategies that require the maximization of oil production in a short development cycle, chemical flooding can become a potential avenue to accelerate oil production in secondary oil recovery mode. This makes it different from onshore chemical flooding processes that mostly focus on enhanced oil recovery in mature or maturing reservoirs. The advancements in offshore chemical flooding field applications are reviewed and analyzed. By summarizing offshore application cases, the presented analysis also assesses the chemical formulations applied or studied and injection/production facilities required in offshore environments. The main technical challenges are also discussed for scaling up the applications on offshore platforms or floating production storage and offloading (FPSO) systems. The chemical flooding technologies reviewed include polymer flooding, surfactant-polymer (SP) flooding, and alkaline-surfactant-polymer (ASP) flooding. By assessing the technology readiness level of these technologies, this study presents their perspectives and practical relevance for offshore chemical flooding applications. It has been long realized that chemical flooding, especially polymer flooding, can improve oil recovery in offshore oil fields. The applications in Bohai Bay (China), Dalia (Angola), and Captain (North Sea) provide the know-how workflows for offshore polymer flooding from laboratory to full-field applications. It is feasible to implement offshore polymer injection either on a platform or in an FPSO system. It is recommended to implement polymer flooding at an early stage of reservoir development to maximize the investment in offshore facilities. By tuning the chemistry of polymer products, they can present very good compatibility with seawaters. Therefore, choosing a proper polymer is no longer a big issue for offshore polymer flooding. There are also some interesting findings reported on the development of novel surfactant chemistries for offshore applications. The outcome from a number of small-scale trials, including the single-well chemical tracer tests on surfactant, alkaline-surfactant (AS), and SP in offshore Malaysia, Abu Dhabi, Qatar, and South China Sea, provided valuable insights for the feasibility of chemical flooding in offshore environments. However, the technology readiness levels of surfactant-based chemical flooding processes are still low, partially due to their complex interactions with subsurface fluids and the lack of interest in producing residual oil from matured offshore reservoirs. Based on the lessons learned from offshore applications, it can be concluded that several major challenges still need to be overcome in terms of large well spacing, reservoir voidage, produced fluid treatment, and high operational expense to successfully scale up surfactant-based chemical flooding processes for offshore applications. Key Terms and Phrases offshore chemical flooding; polymer flooding; surfactant-based chemical flooding; offshore application cases; lessons learned

Evaluation of the Kinetic and Thermodynamic Behavior of Tracers for Their Applicability in SWCTT

Determining residual oil saturation by the single-well chemical tracer test (SWCTT) is of key importance for assessing the potential of enhanced oil recovery (EOR) and developing EOR pilot projects. However, the test trials conducted since the first injections of tracer compositions until now have not resulted in a detailed analysis of the selection of candidates for single-well tracers and their limits of applicability in various reservoir conditions. The purpose of this study was to consider the influence of the structure on the kinetic and thermodynamic components of tracers to assess their application’s operating intervals. It is shown that the rate of single-phase and two-phase hydrolysis of the primary partitioning tracer makes it possible to predict the shut-in time by calculating when the tracer is injected at the reservoir temperature. The influence of the tracer structure during the extraction process with an increase in the hydrocarbon chain of the ester in a different range of brine salinity and temperature has been studied. As a result, this work provides a method for evaluating the thermodynamic and kinetic behavior of primary tracers to establish minimum and maximum threshold K-values at various values of residual oil saturation, temperature, and brine salinity, taking into account the optimal time of the well shut-in to carry out at least 1/2 hydrolysis of esters.

The results of single well chemical tracer tests to assess the effectiveness of surfactant-polymer exposure at the Kholmogorskoye field

Methods of enhanced oil recovery in general and surfactant-polymer flooding in particular are considered as tertiary methods for the development of mature oil fields in Western Siberia, with the potential to increase oil recovery to 60-70% of the initial geological reserves.
 To select an effective mixture of surfactants and polymer for surfactantpolymer flooding, laboratory tests were carried out in which the thermal stability, phase behaviour, interfacial tension and rheology of the formulations were tested. Filtration experiments were also carried out to optimize the volumes of injected fringes and the concentrations of reagents in them. At the final stage, single well chemical tracer tests (SWCTT) were carried out to assess the effectiveness of surfactant-polymer flooding on two wells of the Kholmogorskoye field. In order to investigate different technical and economic models of surfactant-polymer exposure, SWCTT tests were conducted with the same surfactant, but with a different design. The results of the SWCTT tests showed that the residual oil saturation in the affected area after injection of the surfactant-polymer solution decreased by about 11% compared to water flooding, which is about a third of the residual oil after flooding. The tested surfactant showed acceptable efficiency under suboptimal temperature conditions, which is favorable for the use of the selected surfactant-polymer composition for neighboring deposits and formations with different reservoir temperatures, but similar water composition.
 In general, the results of the conducted field tests correlate with the results of the main laboratory experiments for the selected surfactant.

New and simple methods of determination partition coefficient and degree hydrolysis of tracer for estimating residual oil saturation by SWCTT technologies

SWCTT (Single-Well Chemical Tracer Test) actively used to determine the reservoir oil saturation as cost effective and quick method. To use this technology, the partition coefficients K and hydrolysis degree H of the tracer which determined experimentally in laboratories are essential for calculation of residual oil and estimation shut-off period of the well. In this paper, FTIR-spectroscopy and pH-metry were proposed for determination of partition coefficient of chemical tracer (ethyl acetate) and hydrolysis degree. Results obtained by these methods showed good correspondence with the data obtained by standard gas chromatographic analysis that confirms its applicability. All of these methods were used for the study of SWCTT parameters for four oilfields with different formation temperatures in the range 27–70 °C and formation water salinities up to 215,015 ppm. It was shown that oil-water partition coefficient of partitioning tracer increases with the rise of salinity. The apparent activation parameters of the hydrolysis reaction were determined by data on the degree of hydrolysis (pH-metry) and the concentration changes of tracer (head space gas chromatography analysis).

Experimental investigation of surfactant polymer injection for improving enhanced oil recovery in designed-micromodel and y-field

Due to the remarkable oil reduction in oil fields, enhanced oil recovery (EOR) techniques have been considered by a large number of scientists. In this study, the surfactant-polymer injection(SP) was firstly tested in a designed-micro model by using sand, gasoil and a crude oil to evaluate its performances, and then it was applied in the Y- field by involving a Single Well Chemical Tracer Test (SWCTT) to determinate the residual oil saturation (Sor) and the ability of the field to EOR. The results indicate that SP-flooding leads to the recovery of 95% of residual oil contained in 5 m radius.

A Single Well Chemical Tracer model that accounts for temperature gradients, pH changes and buffering

We present the first ever Single Well Chemical Tracer (SWCT) model that incorporates temperature gradients and pH-driven ethyl acetate hydrolysis rate changes when buffering by either calcite in the oil-bearing formation or by chemical species in the injected brine are accounted for. The model is applied to four generic cases with SWCT test, rock formation and brine composition data based on published values. An analytical model is used to calculate how the brine injection temperature decreases with time and an axially symmetric numerical model is used to simulate the cooling of the oil-bearing formation, chemical reactions and transport of species in the reservoir. The primary oil-water partitioning tracer ethyl acetate hydrolyses into the secondary water tracer ethanol and acetic acid that lowers the pH and changes the hydrolysis rate. The acid driven pH decrease may be significantly reduced by pH buffering mechanisms. We investigate four different buffer models. Model 1 may represent a sandstone with calcite cement. In Model 2, also with calcite cement, the brine has less calcium but more bicarbonate and initially lower pH than Model 1. Model 3 is a clean sandstone without calcite cement and the brine contains no calcium but much bicarbonate. Model 4 has no buffer capacity neither in the brine nor in the target formation. Although we discuss only four models, they are quite different and represent an important subset of the realistic SWCT test parameter space. A temperature gradient develops across the primary tracer bank in all four models and this causes the secondary tracer to be displaced away from the wellbore relative to the primary tracer. We find that most of the ethyl acetate hydrolysis takes place during shut-in. The residual oil saturation (Sor) is estimated from the synthetic tracer production curves using the simple chromatographic separation equation as well as with the mean residence time correction. The two methods underestimate the true Sor value (22%) by 3–5 and 3–11% (saturation points), respectively. These results demonstrate that the ‘handicap’ created by the temperature gradient together with the pH-driven hydrolysis rate changes should be taken into account in Sor estimates. We suggest that SWCT tests may be used to evaluate the effective wettability of the target formation at a modest extra cost. Using water tracers with very low quantification limits as primary tracers should reduce or even eliminate the problems caused by temperature gradients and pH-driven hydrolysis rate changes since they will travel well ahead of the temperature gradient and the small amount of acid generated will produce only an insignificant reduction in pH.

Hydrocarbon saturation determination with the single-well-chemical-tracer-test under laboratory conditions

Nowadays the share of hard-to-recover reserves is growing, and to maintain oil production on necessarily level, we need to involve hard-to-recover reserves or to increase oil production efficiency on a brownfields due to enhanced oil recovery. The efficiency of enhanced oil recovery can be estimated by oil saturation reduction. Single-well-chemical-tracer-test (SWCTT) is increasingly used to estimate oil saturation before and after enhanced oil recovery application. To interpret results of SWCTT, reservoir simulation is recommended. Oil saturation has been calculated by SWCTT interpretation with use of reservoir simulator (CMG STARS). Distribution constants has been corrected due to results of real core sample model, and core tests has been successfully simulated. Obtained values of oil saturation corresponds with real oil saturation of samples. Thus, SWCTT as a method of oil saturation estimation shows good results. This method is promising for enhanced oil recovery efficiency estimation.

The prospects and potential opportunities of low salinity water flooding for offshore applications in sandstones

Low salinity water flooding (LSWF) is an emerging enhanced oil recovery (EOR) technology with enormous potential for offshore applications in sandstones. The objective of this study is to provide a review on LSWF offshore field applications and summarize the key lessons learned. A review was also conducted on the capabilities of existing sulfate removal units (SRUs) for seawater injection in offshore fields. Furthermore, the potential of targeting offshore oil fields with de-sulfated seawater (DSSW) injection, either ongoing or planned, as primary candidates to switch over to LSWF EOR has been investigated.For LSWF field trials, the chance of success can be significantly improved when it is based on key laboratory screening tests such as reservoir condition corefloods. The LSWF field tests implemented in offshore fields mainly comprise of single well chemical tracer tests (SWCTT) and interwell trials. However, the reported interwell field trials are restricted to unconfined pilots, which makes the injection and production allocation difficult. Therefore, confined pilots are recommended for future consideration of LSWF field trials to provide better estimations on swept volumes and accurately determine the improvements in the displacement efficiency.Globally there are more than 80 sulfate removal units in operation for de-sulfated seawater (DSSW) injection in offshore waterflooding. All these offshore fields with DSSW injection either currently ongoing or planned can become potential candidates for screening and switch over to LSWF EOR for the following two reasons: (1) The divalent cations can act as bridges between negatively charged rock surfaces and negatively charged polar oil components to increase the oil-wet tendency. These bridges can become primary targets to be replaced by un-complexed cations in low salinity water for EOR. (2) The de-sulfated seawater injection processes can easily be switched to LSWF by retrofitting the existing sulfate removal units with reverse osmosis (RO) desalination systems and upgrading the facilities to handle larger water volumes. Such retrofitting is considered sufficient to generate low salinity water, which can bring significant gains to increase oil recovery with minimal additional investment. The availability of RO desalination systems on offshore platforms can also provide potential opportunities for the cost-effective implementation of tertiary chemical EOR technologies especially polymer flooding.The novelty of this study is that it provides useful guidelines for reservoir screening and offshore field implementation of LSWF processes. Also, the new potential of evaluating the offshore oil fields with existing DSSW injection for switching over to LSWF has been identified. These findings will have a potential impact on enhancing the LSWF prospects and opportunities for EOR in different offshore fields.

Temperature gradient and pH effects on Sor estimates from SWCT tests - The no buffer case

Residual oil saturation (Sor) is the fraction of immobile oil that remains after a water flood. This information is important for calculating recoverable reserves and evaluating EOR campaigns. Single Well Chemical Tracer (SWCT) tests yield a near-well average Sor based on a large rock volume. Correctly performed and analyzed, SWCT tests yield reliable Sor estimates. There are, however, two important effects that commonly are ignored. One is cooling of the reservoir during injection of the cold brine resulting in decreased hydrolysis rate of the primary tracer. The other is the hydrolysis rate's pH dependence. To study these phenomena, we have developed a numerical model of an ethyl acetate SWCT test and applied it to a generic case with test and formation data based on published values. An analytical model is used to calculate how the brine injection temperature varies with time. The ethyl acetate hydrolyses into ethanol and acetic acid that lowers the pH and changes the hydrolysis rate. Any buffer capacity in the injected fluid or formation is neglected. Our model is the first ever to combine realistic temperature calculations with pH dependent ethyl acetate hydrolysis rate during a SWCT test. Sor is estimated from synthetic tracer production curves using the direct chromatographic separation equation as well as the mean residence time. A large number of simulations were performed to investigate how the Sor estimates vary with different model assumptions and input data. In all cases, both methods underestimate the true Sor, typically by 2–4% - in some cases significantly more. We demonstrate that neglecting dispersion in the wellbore during injection and production is warranted with the rock and test data used in this study.

Experimental design methodology as a tool to optimize the adsorption of new surfactant on the Algerian rock reservoir: cEOR applications

In this research work, a new surfactant called surf EOR ASP 5100 used in the SWCTT (single well chemical tracer test) in the Algerian oilfield and sodium dodecyl sulfate (SDS) were used for static adsorption tests. The Algerian rock reservoir has been characterized by different techniques such as SEM, XRD, XRF, BET analysis. The equilibrium was successfully verified by Langmuir isotherm and second-order ( $R^2>95\%$ models for all concentrations and temperatures to predict the adsorption process. Furthermore, the adsorption process was found to be exothermic ( $\Delta G^{\circ}<0$ . To quantify the minimal adsorbed quantity, a full factorial design of 23 (8 experiments) was applied to analyze the individual effects and interactions of operational parameters using variance analysis (ANOVA), desirability method and response surface methodology. The optimal conditions obtained are as follows: the Qe value was 2.3291mg/g for the SDS surfactant at a concentration of 200ppm and temperature of $ 25 {}^{\circ}$ C, and Qe was 3.894513mg/g for EOR ASP 5100 for the concentration of 200ppm and temperature of $ 80 {}^{\circ}$ C.

Improved Method of Moment To Determine Mobile-Phase Saturations Using a Single-Well Chemical-Tracer Test

Summary The single-well chemical-tracer test (SWCTT) has been applied for decades with much success in estimating residual oil saturation in near-wellbore locations. The information obtained from an SWCTT is critical for designing a method for enhanced oil recovery (EOR). However, a key assumption in the conventional SWCTT is that only single-phase (water) is mobile. In reality, this is often not the case, and significant error can occur if the conventional SWCTT analysis method is used when multiple phases flow at the same time. The objective of this study is to improve the accuracy and precision of SWCTT interpretation in a multiphase-flow condition. In this paper, we propose an innovative procedure of SWCTT and modify the method of moment (MOM), aiming at the two-mobile-phase condition. In the development of the algorithm, a ratio parameter is introduced to adjust the calculated swept-volume difference between the conservative tracer and the partitioning tracer. In addition, a mixture injection of oil and water is required, instead of the pure-water injection in an SWCTT. The proposed approach is verified through numerical simulation on synthetic cases with known input parameters. The model being simulated consists of a radial-flow regime with a single vertical well in the center. The input oil saturation varies from 0.1 (immobile oil saturation) to 0.9. Our results show that the saturation estimated from the modified MOM matched the simulation input data, indicating that our approach is able to capture the saturations under two-mobile-phase condition. Moreover, the modified MOM can also be applied in single-mobile-phase condition and can improve the accuracy of conventional MOM.

Surface-Charge Alteration at the Carbonate/Brine Interface During Single-Well Chemical-Tracer Tests: Surface-Complexation Model

Summary Water chemistry has been shown to affect oil recovery by affecting surface charge and rock dissolution. The single-well chemical-tracer (SWCT) test is a field method to measure residual oil saturation (Sor), in which hydrolysis reaction of an ester has been known as a key process that could displace the equilibrium state of a reservoir by changing formation-water (FW) composition. Because oil mobilization during the SWCT tests causes an error in the measurement of Sor, changes in water chemistry might be a concern for the accuracy of Sor measurements. In our previous work, the extent to which different reservoir parameters might change water composition and the effect of water-chemistry changes on the calcite dissolution and the oil liberation from the carbonate-rock surfaces were extensively evaluated. In this study, the effect of water-chemistry changes on surface-charge alteration at the carbonate/brine interface has been studied by constructing and applying a surface-complexation model (SCM) that couples bulk aqueous and surface chemistry. We present how the pH drop induced by the displacement of the equilibrium state and changes in water chemistry in the formation affect surface charge in a pure-calcite carbonate rock during the SWCT tests. The results show that a pH drop during the SWCT tests while calcium concentration is held constant in the FW by ignoring calcite dissolution yields a less-positive/more-negative surface charge so that wettability of carbonate rock might be altered to a less-oil-wetting state, when the oil is negatively charged. In reality, however, calcite dissolves by water-chemistry changes during the SWCT tests, which leads to an increasing calcium concentration in the FW. Consequently, an SWCT test in carbonates is accompanied by increasing calcium concentration while pH drops, which yields an increase in the surface charge of carbonate rocks. Therefore, the pH drop does not directly affect the surface charge of carbonate rock during an SWCT test, and calcium concentration increased from calcite dissolution could control the surface charge more significantly.

A Step Change for Single-Well Chemical-Tracer Tests: Field Pilot Testing of New Sets of Novel Tracers

SummaryThe single-well chemical-tracer test (SWCTT) is an in-situ test to measure oil saturation, and has been used extensively to assess the potential for enhanced oil recovery (EOR) or to qualify particular EOR chemicals and methods. An SWCTT requires that a primary tracer be injected and that a secondary tracer be generated from the primary tracer in situ. Typically, a few hundred liters of ester is injected as primary tracer, and the secondary tracer is formed through hydrolysis in the formations. The ester is an oil/water-partitioning tracer, whereas the in-situ-generated alcohol is a water tracer. During production, these tracers separate and the time lag of the ester vs. the alcohol is used to estimate oil saturation in the near-well region.In this paper, we report a field test of a class of new reacting tracers for SWCTTs. In the test, approximately 100 cm3 of each of the new tracers was injected and used to assess oil saturation. In the test, ethyl acetate (EtAc) was used as a benchmark to verify the new tracers. This paper reviews the design and implementation of the test, highlights operational issues, provides a summary of the analyzed tracer curves, and gives a summary of the interpretation methodology used to find oil saturations from the tracer curves. Briefly summarized, we find the Sor measured by each of the novel tracers to compare with that from a conventional SWCTT. To validate stability and detectability of the tracers, a mass-balance assessment for the new tracers is compared with that of the conventional tracers.A benefit of the new tracers is the small amount needed. Methodological advantages resulting from using small amounts include the possibility to inject a mix of several tracers. Using several tracers with different partitioning coefficients enables probing of different depths of the reservoir. In addition, the robustness of SWCTTs can be increased by using several tracers, with different reaction rates and temperature sensitivity. The field trial also demonstrated that the new tracers have operational advantages. One benefit is the possibility to inject the new tracers as a short pulse of 10 minutes. Other benefits are that the small amounts needed reduce operational hazards and ease logistical handling.

Significance of Geochemistry in Single-Well Chemical-Tracer Tests by Coupling a Multiphase-Flow Simulator to the Geochemical Package

Summary Single-well chemical tracer (SWCT) is the most commonly used field method to determine oil or water saturation in one-well enhanced-oil-recovery (EOR) pilots. Because hydrolysis of an ester, which is the main part of the method, leads to forming acid as well as alcohol, the equilibrium state of the reservoir is disturbed, and thus the pH changes. It is generally accepted that the hydrolysis-reaction rate is mainly dependent on the pH and temperature. Therefore, it is required to know the extent to which this dependency might affect the shape of the product-tracer profiles and the numerical interpretation of the field-test data for computing residual oil saturation (Sor). In this study, this notion has been investigated by coupling a multiphase-flow simulator to the geochemistry package PHREEQC (Parkhust and Appelo 2013). In this study, the PHREEQC geochemical simulator has been used to illustrate the extent to which different parameters might affect the pH variation during the test. The PHREEQC database has been modified to take the ester-hydrolysis reaction into account by adding the ester, alcohol, and acid-product species. The hydrolysis-reaction mechanisms of ester have also been programmed to account for the dependency of the hydrolysis reaction on the pH. Also, because ester partitions into the oil phase and travels behind the water phase (i.e., Darcy velocity), performing two-phase flow would be necessary to highlight the significance of the pH dependency of the hydrolysis-reaction rate on the tracer profiles. For doing that, a multiphase Buckley-Leverett (BL) flow simulation is coupled with IPhreeqc, which is an open-source module of the PHREEQC geochemical package. Then, a California Turbidite SWCT test has been re-evaluated to verify the approach. At the end, the geochemistry of a reservoir with an almost weak resistance (high temperature and weak buffer capacity) against pH variation in the SWCT test has been studied using the geochemical-based approach. The results show that the variation of the hydrolysis rate with pH could affect mainly the tail edge of the predicted tracer profiles, and it could marginally affect the apex of the profiles; however, it might affect the interpreted value of the Sor measurement as the resistance against pH variation becomes weaker. In these conditions, adapting the SWCT-test designs (i.e., shut-in time and injecting lower concentration of ester) could diminish the pH variation. The pH dependency of the hydrolysis-reaction rate is recommended for the numerical interpretation of the field SWCT-test data. The results of this study can be used to minimize the uncertainties of the SWCT tests and to improve the reliability of the Sor measurements.

Studying the Potential of Calcite Dissolution on Oil Liberation from Rock Surfaces during Single-Well-Chemical-Tracer Tests by Coupling a Multiphase Flow Simulator to the Geochemical Package

Calcite mineral dissolution has been considered to be an important mechanism for dampening the considerable pH-variation during Single-Well-Chemical-Tracer (SWCT) tests by improving the buffer capacity of the aqueous solution. Other parameters that also could have a great effect on the geochemistry of the reservoir during the SWCT tests are water buffer capacity, soluble hydrocarbon components, and temperature. Additionally, calcite mineral dissolution has been also presented over the last decade as an underlying mechanism for liberation of the adsorbed oil from the surface by modified salinity water injection (MSWI) in carbonate reservoirs. This contradiction of effects of calcite dissolution might pose a challenge for the accuracy of the SWCT tests in carbonates. This concern motivated us to highlight the potential of the calcite dissolution on the oil liberation from the carbonate rock surfaces during the SWCT tests by coupling a multiphase flow simulator to the geochemistry package PHREEQC. The results show that although the calcite dissolution is marginal during injection time, it might be substantial during shut-in time which is much longer. During shut-in time, the results show that the potential of calcite dissolution on the oil liberation from the rock surfaces could be more significant at higher reservoir temperatures although initial solid calcite concentration and buffer capacity also could have an effect. It is also clear that the pH of the system reaches the lowest level when the shut-in time reaches the transient time (i.e., injection and production times) and it is not changed significantly afterwards. At longer shut-in times, the additional ester hydrolysis and acid product is neutralized by the calcite dissolution and the buffer capacity of water. Therefore, the probability of the liberation of the adsorbed oil from the rock surface is higher at larger shut-in times so that test designs with shorter shut-in times and even as short as the transient time for the carbonate reservoirs is highly recommended. We hope that this study can be used to minimize the uncertainties of the SWCT tests and improve the reliability of the Sor measurements.

Pushing the envelope of residual oil measurement: A field case study of a new class of inter-well chemical tracers

The success of any improved oil recovery (IOR) project is largely dependent on how much oil is remaining to be mobilized within the targeted area of the partially depleted or mature reservoir. Partitioning tracers are generally used to measure residual oil saturation (Sor) or remaining oil saturation (ROS) in the near wellbore region via a single well chemical tracer test (SWCTT) or in an inter-well region via a partitioning inter-well tracer test (PITT). There is a limited repertoire of nonradioactive and environmentally friendly inter-well partitioning tracers for measuring ROS. A new class of environmentally friendly partitioning tracers was field tested, in a giant carbonate reservoir undergoing peripheral waterflood, for measuring ROS in inter-well regions in a depleted area.The new partitioning tracers were qualified via laboratory experiments and are deemed to be very stable at reservoir conditions (213 °F and a salinity range of 60–200 kppm). The field pilot was conducted concurrently with a set of non-partitioning inter-well chemical tracer test (IWCTT) to determine reservoir connectivity, water breakthrough times, and injector-to-producer pair communication in an area selected for an IOR/EOR field pilot. An elaborate sampling and analysis program was carried out over a period of 30 months.This paper reviews the complete design and implementation of the test, operational issues, and the analyses and interpretation of the results. The breakthrough times of the passive and partitioning tracers are reported, and inter-well connectivity between the paired and cross-paired injectors and producers are analyzed. The ROS measured by a majority of the novel tracers is comparable to the saturations obtained via SWCTT, core and log derived saturations.The combination of conventional IWCTT and the novel partitioning tracers via PITT has been very useful in analyzing well interconnectivity, understanding the reservoir dynamics and quantifying remaining oil saturation distribution in the reservoir. This has led to better reservoir description and an improved dynamic simulation model.

Effect of Reservoirs Conditions on Designing Single-Well Chemical Tracer Tests Under Extreme Brine Conditions

The single-well chemical tracer test (SWCTT) has emerged in the past decades as a method for measuring oil saturation prior to and/or after EOR operations, to measure the recovery performance in-situ. To use this technology, the partition coefficients of the selected tracers are essential for estimating the level of residual oil at the targeted single well. Commonly, injection of short chain alcohols and ethyl acetate, a reactive tracer, is carried out for the tracer slug, mainly based on site-specific reservoir conditions, to accurately determine the level of oil saturation in-situ. However, injection of ethyl formate has been less common due to its fast hydrolysis rate under elevated temperature, which increases the challenges in data interpretation. Therefore, a systematic study for using ethyl formate under mid-range temperature $$(<60\,^{\circ }\hbox {C})$$ , as commonly found in mature oil field in the USA, shows the potential to be applied for SWCTT. As part of the design effort for a series of EOR field tests to manage the project risk, we particularly assessed the relationships between the partition coefficients of reactive tracers and subsurface conditions such as salinity, temperatures, type of electrolytes, and the equivalent alkane carbon number (EACN) of the crude oil experiments was performed under various reservoir conditions as a function of actual site characteristics at the targeted high saline formations. In brief, our data clearly show that the (oil/water) partition coefficient of ethyl formate increases steadily with increasing NaCl concentrations, ranging from 10,000 (0.17 M) to 250,000 mg/L (4.28 M). A similar upward trend was observed for increasing temperature between 25 and $$52\,^{\circ }\hbox {C}$$ ; however, the partition coefficient decreases inversely with increasing the crude oil EACN over the range from 8 to 12, which are common for domestic oil samples. It was also showed that brine with high NaCl concentration yielded higher partition coefficients. In contrast, brine with high $$\hbox {CaCl}_{2}$$ and $$\hbox {BaCl}_{2}$$ concentration yielded lower values. And $$\hbox {MgCl}_{2}$$ performed somewhat unusual trend in our tests. These results further indicate that the partition coefficient of the reactive tracer, ethyl formate, is sensitive to change in salinity, temperatures, type of electrolytes and EACN, as observed for other chemical tracers. In addition, based on the hydrolysis rate of ethyl formate under various reservoir conditions, the appropriate window of shut-in time can be pre-determined before initiating the field test. We believe that the ability of better understanding the partition coefficients and predicting the shut-in time beforehand could drastically reduce the risks of SWCTT operations.

A comprehensive framework for the theoretical assessment of the single-well-chemical-tracer tests

Single-well-chemical-tracer (SWCT) is the most commonly used field method to determine oil or water saturation in one-spot pilot. This method is a complex process due to many effective parameters and non-ideality factors involved. Understanding the extent to which theses parameters might affect the SWCT test profiles could help us to manage and design the SWCT test more efficient at different reservoir conditions. This paper proposes a comprehensive framework of a new approach to highlight different aspects of the SWCT tests theoretically before implementing the field test. In order to accomplish the task, combining of numerical and analytical solutions have been used.The devised algorithm has been programmed in six different stages. In the first four stages, all test design parameters in different investigation regions and retardation factors are calculated. The test design parameters are sizing the test volume, test timing (i.e., injection, shut-in, and production), tracer concentration during the test, and the mean residence volume. In the fifth stage, all criteria are taken into consideration to find the most efficient test designs. Then, the achieved parameters are applied in the simulation stage (sixth stage) to investigate the effect of the ester bank and concentration, ester properties, shut-in time. The geochemical speciation code PHREEQC is also used to study the level of pH-variation during shut-in time. The effects of the calcite dissolution, temperature, and initial buffer capacity have been evaluated on the probability of pH-variation. The results show that the probability of pH-variation increases with temperature and lower amount of calcite concentration.To evaluate the workflow methodology, two different field test cases with different reservoir conditions are employed in order to reflect the influence of different stages of the algorithm. We hope that the workflow developed can be used to minimize the uncertainties and improve the quality of the SWCT tests.

Single-Well Chemical-Tracer Modeling of Low-Salinity-Water Injection in Carbonates

Summary The interest in low-salinity-water injection (LSWI) compared with seawater injection or high-salinity-produced-brine injection is increasing in both laboratory and field tests. The single-well chemical-tracer test (SWCTT) is also becoming increasingly popular as an in-situ test to assess the reduction in oil saturation caused by an enhanced-oil-recovery (EOR) process. Hence, accurate modeling of SWCTTs is essential. In this paper, modeling and simulation of the SWCTT of LSWI in a carbonate reservoir is investigated by use of the UTCHEM reservoir simulator, a nonisothermal, 3D, multiphase, multicomponent, chemical compositional simulator developed at the University of Texas at Austin (UTCHEM 2011). Both radial- and Cartesian-grid models are set up for a field-scale pilot by use of measured rock and fluid data of a Middle Eastern reservoir. Tracer reactions and the empirical LSWI model implemented in UTCHEM are used to estimate residual oil saturation (ROS) as a result of LSWI. Two approaches are used to estimate ROS to LSWI, including analytical and numerical methods. Results show that both approaches give consistent values for ROS for homogeneous radial- and Cartesian-grid models. The two approaches were inconsistent for the multilayer radial model, which highlights the necessity of the use of numerical approaches for layered reservoirs. The Cartesian-grid model was used to investigate the effect of heterogeneity on SWCTT results, where a new numerical approach is proposed for estimating ROS. This finding validates the approach used and the implementation of both tracer reactions and the LSWI model in UTCHEM. The proposed approach can now be used to estimate ROS of the SWCTT for reservoirs with different degrees of heterogeneity, which provides a clear insight into reservoir performance before planning multiwell demonstration pilots.