In Argentina, unconventional reservoir development learning processes have been continuously advancing, specially lowering project costs in Vaca Muerta's different zones, light oil, gas-condensate and dry gas. However, the actual international oil and gas prices are forcing companies to search for additional improvements to keep on developing these fields. Vaca Muerta Oil and gas potential with big pay thickness requires the review of completion efficiency and its incidence on initial hydrocarbons production in order to improve the project cash flow. The analysis should be focused on horizontal wells-orthogonal fractures interface which, in Argentina, has not been properly evaluated, leading us to underestimate the production loss by using the status quo of approved technologies for our projects. Big thicknesses increase the pressure loss effects due to flow convergence from the fracture into the well, to which we have to add the near wellbore bottleneck causing a conductivity reduction due to inappropriate final concentration design, over flush and proppant grain size and quality that are not capable of holding the confining pressure, in special in some of the completion technologies. In dry gas and gas-condensate reservoirs, non-Darcy effects and condensate accumulation in the SRV when pressure drops below the dew point, are also influenced by an inappropriate completion. Vaca Muerta over pressure forces us to pay attention to plays whit high production due better reservoir characteristics and SRV quality induced by fracturing. Interfaces must be designed to support such high pressures and maintain a good fracture conductivity. Technology review and selection is strongly recommended, in special in those where treatment is divided into multiple fracturing in different clusters with over flush to clean the well and/or displace guns/plugs and ball fractures between stages. Benefits from obtaining bigger and more stable SRV through new microproppant technologies and longer horizontal wells, will not be appreciated if fractures-wellbore interfaces are not improved allowing a better fluids evacuation into the wellbore by reducing restrictions to flow. At the same time, available literature shows dissimilar results when looking to improve cash flow through refracturing depleted wells. The purpose of this article is to show the importance of including the evaluation of production loss magnitude and its incidence on the project cash flow. This will allow considering different completion strategies on each project, some of them already available. Changes in fracture design and proppant quality will lead to a better and more stable fracture-wellbore interface.
TX 75083-3836 U.S.A., fax 1.972.952.9435. AbstractThe objective of this paper is to summarize the future application, limitations, advantages and disadvantages of UBD technology, based on eleven wells that were drilled in this type of reservoir. The geographic location of the wells is the Center of the Neuquen Basin, located in the state of Neuquen, Argentina. The objective was to define the reliable range of the stratum flow capacity value or permeability, in order to forecast the reservour productivity with UBD Technology, assuming the absence of formation damage (skin). The method used was based on the transient-state production equation theory. Implementation was based on the drilling hydraulics design: maintaining constant Pwd close to the reservoir of interest formation pressure; stopping drilling to allow the development of transient production; establishing drilling conditions and detecting the production contributions in new stratums to be evaluated; determining the average production; correlating detected inflows with the pressures obtained by formation testing. The method, together with other data, allowed us to estimate the structure potential productivity, despite the poor petrophisycs characteristics (Tight gas reservoirs) and to demonstrate that the results during completion could give reliabie properties estimatations used in the forecast.
TX 75083-3836 U.S.A., fax 01-972-952-9435. AbstractTo maximize the production and enhanced total recovery of hydrocarbons, the oil companies must have a complete understanding of the reservoir rocks and fluids present in their producing fields. The measurement of rock properties enables us to determine its ability to allow fluids to flow and understand the way these fluids will behave when the reservoir is produced. This information provides the starting point to conduct a Formation Damage Analysis that will lead to an appropriate diagnosis in order to select the best solution. It will provide detailed explanation to the problems that well and production have and will offer basis and support to the stimulation treatment investigation and its effects. The Formation Damage is any mechanical, chemical, biological or thermal process in a reservoir that causes a reduction in porosity and permeability. Almost all the field operations constitute a potential source of damage for the well productivity that often results in reduced productivity of oil or gas reservoirs or reduces injectivity of injection wells, on a secondary recovery, This paper is an attempt to supply a preparatory methodology to enable the potential risks of formation damage evaluation, trying to correlate the results in a laboratory scale with the field test. The laboratory diagnosis, usually preliminary for stimulation, help auspiciously to avoid or decrease the risk of damage. The study and control of formation damage requires a test design consistent with the feasibility of the operatives options and involves properties such as the knowledge of geological and petrophisics of the formation to be stimulated, fluids compatibility, operative procedures, etc.
After almost 200 wells drilled in Cerro Fortunoso and more than 500 in the Barrancas and Ugarteche oilfields, located in Mendoza, Argentina, during the past 20 years, ongoing efforts are dedicated by operator and service companies to solve frequent communication between perforations (CBP) problems. Drilling parameters, well cleaning systems, cement slurries designs and acid stimulation treatments are some of the main aspects that have been reviewed and modified in order to solve this problem. However, random CBP keeps occurring during well completion jobs even when good cement bond logs (CBL) are achieved. The CBP events are typically observed on perforated zones that are separated 10 to 50 m between them, and usually occur either when performing swabbing, while pumping pre-flushes or during the acid treatments. These wells are subjected to high stresses generated by Andina tectonic. Therefore, wells are drilled with deviated trajectories usually up to 40° inclination angle (Cerro Fortunoso). In order to understand the CBP mechanism several studies were conducted. Image logs to obtain geomechanical properties as well as pumping pressure records and falloff profiles during communication events were analyzed. Interactions between the formation and completion fluids, as well as mechanical stresses numerical simulations of the open-hole and cemented-cased near wellbore conditions were conducted. The experimental tests performed with rock samples and completion fluids showed no significant interactions that could cause channeling due to formation shrinkage effects. On the other hand, the numerical simulations studies confirmed the data obtained from image logs and geo-mechanical studies, where high shear stresses generate wellbore oversize calipers and breakouts. The simulations also show that high stress levels are transmitted to the cement and the casing, reaching, in some cases their colapse limit. During the last year, CBP problems were diminished by introducing slight changes in the well design, mud density window and by improving casing centralization. Based on the numerical simulation results, the next step consists in using enhanced cement slurries designs with improved mechanical properties, in order to allow near wellbore stress redistribution. All these changes will avoid formation breakouts and cement rupture that could create fractures along the well trajectory, and therefore, will reduce the possibility for fluid communication during well completion jobs. Introduction CBP strongly affects stimulation treatments efficiency, generates difficulties on formation evaluation and may cause severe production losses due to water communication, as well as an unsuccessful secondary recovery, due to inappropriate water flooding. Several oilfields located in Mendoza state, such as Cerro Fortunoso, Barrancas and Ugarteche, have shown CBP problems since the mid 80´s. These events were particularly observed during acid stimulation treatments. This paper summarizes the results of an extensive study conducted between operator and service company in order to understand the CBP mechanism of these wells and determine the possible solutions to this problem. CBP problems in Mendoza oilfields After almost 200 wells drilled in Cerro Fortunoso and more than 500 in the Barrancas and Ugarteche oilfields, Mendoza, during the past 20 years, ongoing efforts are dedicated by operator and service companies to solve frequent communication between perforations problems. In the past two years a program was carried out were different aspect associated to well drilling and completion were analyzed in order to determine the causes and possible solutions to these problems. During the first 10 months of year 2004, fifty acid stimulation treatments were done, 12 of these presented CBP events. The communications occurred randomly, some of these during acid displacement, others before the acid had reached the formation and others even during previous swabbing operations.
The objective of this work is to show that the estimated stimulate reservoir volume (SRV) from micro seismic analysis under dynamic conditions not always represents the SRV acting as hydrocarbon collector and need to be calibrated with total organic content (TOC), vertical and areal distribution. Furthermore, unlike conventional reservoirs, the potential leakoff damage caused during the fracturing and the SRV generation process, are not detectable by pressure transient analysis (PTA), but us a change on SRV transmissibility. Integrating PTA, net pressure behavior during fracture treatment and micro seismic analysis results, allow us to better understand the reason that seismic events from one or more monitoring wells, might conduct us to wrong interpretation of the active SRV. Field cases show how PTA conventional technique helps to identify the existence or absence of an effective SRV connected to each individual hydraulic fracture stage and its associated best quality TOC thickness. Results confirm that active SRV, generated by hydraulic fracture treatment in lower Vaca Muerta, where the seismic events practically are no detected, PTA analysis has an excellent infinite conductivity fracture behavior on the best TOC thickness. Production potential depends on different factors, net pay with TOC percentage greater than 2.5 %, vertical and areal distribution associated to rock porosity, natural fissured and SRV are the most important parameters. It is critical to understand the system deliverability potential, it must be evaluated under production conditions and the active SRV adjusted with TOC percentage and distribution, taking into account the effects caused by SRV pore pressure drop. Network of natural fissures originally in place and changes generated around the main fracture, combine to enhance the quality system. An improvement of active SRV transmissibility was detected after the buildup period, increasing from 20 to 80 % oil production and more than three times the initial production. As in conventional reservoirs, where pressure test results are related to formation transmissibility, in unconventional reservoir pressure test results are related to SRV transmissibility and TOC percentage. It can be concluded that the best zones to be fractured are determined by PTA associating to logging and cutting geochemistry analysis. PTA evaluation will determine only chocked fracture damage and not surface fracture damage. PTA allows us to improve well completions efficiency and amount of fractures. Vertical wells production performance and best zones knowledge, will allow a better choice of zones to be developed thru drilling horizontal wells. In spite of industry belief, PTA analysis can give us a better understanding of SRV behavior in contact with the generated primary fracture.
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