The recoveries from the wells in an oilfield do not usually form a perfectly lognormal distribution. So the log probability plots of the recoveries usually do not exactly conform to the straight line that signifies lognormally distributed data. Certain types of deviation from the straight line can provide useful information about the oilwell recoveries.
A set of recoveries may differ from the lognormal
and all these cases have characteristic effects on the shape of the log probability plot. Some illustrative field cases are presented.
A quick review of the quality of a set of oil recovery figures can be made by preparing a log probability plot. Certain characteristic shapes of the plot may indicate that the data is incomplete or that it is not homogenous and should be disaggregated for further analysis.
A variable is said to be lognormally distributed if the logarithms of the variable are normally distributed1,2,3. Many reservoir properties have been found to be approximately lognormally distributed, for example formation thickness, core permeabilities, oil recoveries and field sizes1. This paper explores the significance of certain types of deviations from the lognormal.
A log probability plot is similar to a normal probability plot except that a logarithmic scale is used to plot values of the variable. The cumulative frequency distribution of a lognormal distribution forms a straight line when displayed on a log probability plot. But for real oilfields, the oil recoveries usually are not exactly lognormal, and deviate from the straight line.
A plot may deviate from the straight line simply because the data is not exactly lognormal: the distribution may be skewed, with one tail longer than the other, or the peak of the distribution may be higher or lower than the lognormal. But the data may deviate from the straight line because it is incomplete. For example, the field may have had several owners, not all of whom were equally careful about recording oil recoveries. Also, the data may deviate from the straight line because it is not homogenous. For example, if the oilfield includes different reservoirs with very different producing characteristics, then it may be appropriate to separate the oil recovery data into sub groups before making cumulative distribution plots.
This paper presents sets of data which show how some of these types of deviation can be identified from the log probability plot.
Ideal Normal Distributions
Figure 1 shows the frequency distribution of three normal distributions with different means but the same standard deviation. These are examples of the famous bell curve or Gaussian distribution. Since the distributions are symmetric, for each distribution the peak represents the mean. It can be seen that distributions with different means are displaced laterally on the plot. Figure 2 shows the frequency distribution of three normal distributions with different standard deviations but the same mean. It can be seen that distributions with different standard deviations have different widths.
With the main objective of reducing working times of rig equipments and this way costs for operator companies an insertable Progress Cavity Pump (PCP) has been developed. This pump is designed to be anchored in the top (by cups) and with a 2 7/8?? anchor in the bottom section.
This kind of pumps can be installed in wells where beam pumped systems were installed before, and vice versa, avoiding the production string pulling. So if a change from a beam pumped system to PCP system is required, utilizing this new design, the rig activity would be limited to: pulling rod string, changing the pump and installing the string again. If a fast mounting rig is used to realize the replacement, the working time could be shorter.
Other advantage of insertable PCP is that the production flow rates has been increased, obtaining a several range of applications.
This paper presents a short technical description of and some tests of insertable PC Pumps in different wells of Guadales and Lomas Del Cuy oilfields (Area Guadal - Las Heras - Repsol YPF). Theses pumps were tried at different depths, stresses, hydraulic powers and other well conditions in order to demonstrate the same performance of traditional PC Pumps.
An economical analysis exhibits the convenience and some qualitative benefits of installing insertable PC Pumps in a certain range of wells, depending on their flow rates, especially when a beam pumped system will be replaced for a PC Pumped system. Besides it provides a future projection of insertable PC Pumps installations in the mentioned oilfields.
EL GUADAL oilfield is located in Santa Cruz north Argentina area. Its well production is poor so we are forced to make continous analysis in order to reduce costs.
Nowadays, the extraction costs are high so the possibility of changing the elevation system without moving the tubing was analysed. This would allow a lower cost by intervention and a more optimised production due to more well servicings throughout the year.
This document deals with the main economic and technical aspects of the change of the elevation system from mechanic pumping into progressive pumping which decreases the well maneuvre costs in EL Guadal oilfield, Las Heras-Santa Cruz-Argentina.
The introduction of a Flush By equipment would also be helpful because it does not require tubing movement.
The purpose of this paper is to create a calculation model to estimate the operation failure probability of an oil production facility, to guarantee an operation without accidents, without dangerous failures, observing the safety and environment policy established by the company. The facility operation is considered successful when it can carry out the functions it was design for.
Once the failure rate has been obtained, the system reliability level is obtained. The quantification of the success or failure is carried out through the application of the Failure Tree technique. The Safety and Operativity systems have been separately analyzed; taking into account that each of them has to be considered independently.
The model was used in the calculation of facility reliability in an oil field. (1)
In the current maintenance management, the reliability concept has the purpose of creating maintenance plans based on technical criteria that assure the operation of the assets in the different operating contexts.
These concepts are well used in upstream operations, and that is the reason why this model has been developed to incorporate them on the facilities and equipment which, up to now, have been maintained with programs based on strict, periodic maintenance plans, which were made based on personal and manufacturer's experience, or working with the breakage.
Although the typical conditions of this type of industry, and of upstream in particular, make us believe that it would be pointless to work with mathematical and statistical models of calculus, reality shows us that safety and environment conditions currently adopted by companies, governmental regulations and maintenance cost optimization asked for to the operating sectors are the ones that really provide support to the developed calculus.
This developed calculus combines the concepts that are then indicated in such way that they provide a global view of how to obtain the reliability degree in both operation and safety of an upstream oil industry system.
Statement of Theory and Definitions
System: It is a deterministic entity which consists of a collection of discreet elements that interact.
System limit: there are three limits: external limit, internal limit and resolution limit.
External Limit: It is defined by the parts or aspects of the system we want to develop.
Internal Limit: It is defined by the selection of the subsystems we want to divide the system into for analyses purposes.
Resolution Limit: It is the level of a lower subsystem we want to analyze. Fig.1
Safety vs. Control: The safety systems take action when the equipment and process variables reach values out of their safety range. This situation can be due to several reasons, equipment failure (pipeline breakage, stuck valves) abnormal process conditions (abnormal gas rate, low temperatures) and failure of the control equipment itself, operator's mistakes when working with the process in manual, etc.
A particular situation to take into account is the normal or emergency equipment start-up, since the critical variables can move out of the safety range.
The international standards indicate in a compulsory way that the safety systems must be independent of the process control systems, to avoid the failure common causes. The International standard IEC 61508 / 61511 deals with the reliability of the Systems related to safety, linked to the process risks. And only the dangerous failure mode is used.
Simultaneous satisfaction is given to the process safety and continuity.
The use of seismic attributes has increased, especially when extracted from interpreted horizons. The various available attributes are not independent from each other but represent, in fact, different ways of presenting and studying fundamental information from seismic data (time, amplitude, frequency and attenuation). However, statistical analysis using attributes must be based on geological knowledge and not only on mathematical correlation. Petrophysical studies and seismic modeling are sources of understanding. Such knowledge is necessary to improve confidence in observed correlations with reservoir parameters and must be part of all attribute analysis.
However, the use of seismic attributes leads to several questions, for example, what do they all mean? When to use one or another? How to use them on geologic modeling? How reliable those data are? The answers to these questions are not easy, but considering about petrophysical modeling (Porosity, NTG and permeability) what is the best approach: to consider only well data, that are punctual and need to be interpolated, or try to find correlation with physical measurements (seismic data)? Not to consider seismic attributes makes one feel coming back in time, when this important tool was not available.
On a giant oilfield offshore Brazil seismic attributes (‘conventional', complex trace, polynomial decomposition, geometric and coherence) have been used to create geological models and to reduce uncertainties. The attribute choice must be performed by the geophysicist and the geologist working together, in order to check geological meaning of attribute maps, possible physical meaning of the attribute, etc. Plots of the highest correlation values should be visually inspected in order to choose the attribute with best correlation to the desired parameters.
The results show attributes have been favourable to porosity and NTG prediction, but regular (at maximum) to permeability. For permeability even if the results are not so good, the correlation are improving for the latest models (as long as new wells are used). Polynomial decomposion and complex trace attributes have shown better results.
Introduction: seismic attribute definitions and discussions
The use of seismic attribute data for prediction of detailed reservoir properties began more than 30 years ago.
In fact, a seismic attribute is any property derived from seismic reflection signal. Attributes may be compared to lithology in an attempt to devise a method of property prediction away from well control. The method of prediction can vary from a simple linear correlation to multi-attribute analysis, geostatistical methods, etc.
As an evidence of current proliferation the use of attributes, Chen and Sidney (1997) have catalogued more than 60 commom seismic attributes along with a description of their apparent significance and utility.
Although there is a rich history of seismic attributes use in reservoir prediction, the practice remains a difficult and uncertain task. The bulk of this uncertainty arises from the nature of the physics connecting a number of attributes to a corresponding reservoir property. Due to the complex and varied physical processes responsible for various attributes the unambiguous use of attributes for direct prediction will probably remain a challenge for the years to come.
In addition to the fact above described, there is the possibility of coming across statistical pitfalls while using multiple attributes for empirical reservoir property prediction. In addition, many attributes are derived using similar signal processing methods and can, in some cases, be considered largely redundant with respect to their description of the seismic signal.
In mature fields, operators are often seeking ways to increase the hydrocarbon recovery, with the help of reputable service companies. Well stimulation continues to be, by far, the preferred method of achieving such goal. Operators and service companies are continually screening out technologies which will deliver the highest benefit/cost ratio for a particular stimulation well treatment, maintaining focus on operational and health, safety and environment excellencies .
This paper addresses the rebirth of a past hydraulic fracturing technique, born in the 50's, and how it is being successfully applied on onshore mature fields in Brazil: batch fracturing. It is effective due to several technological advancements on proppant density, becoming lighter than conventional frac sand and yet with sufficient mechanical properties to withstand bottom-hole environments. Batch fracturing is now contributing to equally efficient, and more economical well stimulation treatments, providing good economical returns to operating companies.
Batch-Fracturing had limited success in the past. This was due to the available frac fluid and proppant technologies at that time. It is desirable that proppants have low settling when carried by a fracturing fluid, from the time they are added into such fluid, until the end of the pumping process. Batch fracturing applications are on the rise, due to the new families of ultra lightweight proppants, with specific gravities ranging from 1.05 to 1.75. In batch fracturing, the proppant is added to the carrier fluid prepared in standard oilfield mixing tanks,
eliminating the need of specialized mixing equipment such as blenders. Less sophisticated equipment on location implies in lower operational and logistical costs. The carrier fluid ("frac fluid??) does not need to yield high levels of viscosity, and, by consequence, does not have a high load of chemicals (gelling agents, cross-linkers, related breakers…). With batch fracs it is possible to perform common but effective types of fracturing treatments, such as "skin-by-pass?? (a fracture that by-passes the damaged zone), and "partial mono layer' fracturing, both exemplified in this paper, through case histories.
Today, most of the producing oil and gas fields are considered mature. Although continually being redefined, a field is considered mature when its current level of hydrocarbon production has passed its past production peak. Associated with the reservoir's production depletion, there are other hydrocarbon recovery issues inducing operators to continually seek ways to overcome these natural effects. They look, with their subcontracted service companies, for cost effective techniques and technologies able to increase production and
One of the mayor economical impacts in a Project of artificial lift system shift is the associated cost of energy moreover the maintenance and well intervention must be considered. These variables are reflected as addition on the final artificial lift cost selected.
This study was accomplished based on experience at the Teca and Nare fields operated by Omimex Colombia where an artificial lift system shift was performed from Rod Pump (RP) into Progressive Cavity Pump (PCP), achieving significant savings in well downtime and energy consumption at the same volume of production.
The strategy to develop this project started with the identification of well candidates where steam injection was not feasible then a change on the artificial lift system was proposed to a set of wells.
Also is highlighted the importance of the operational variables in long term at the moment to choose an artificial lift system.
The heavy oil reserves have increased more than twice as conventional reserves worldwide. Heavy oil has become in an important issued to the oil industry then and a concern to its best exploitation such technical as economical methods are considered.
Traditionally heavy oil exploitation considered Rod Pump (RP) as artificial lift system, exposing occasionally well downtime as sand stickings and rod failures with poorly designs.
Nowadays thankfully to the technological development an alternative for heavy oil exploitation is presented the Progressive Cavity Pump (PCP) which offers benefits as good heavy oil and high sand contents handling and low initial investment and maintenance cost.
This paper exposes a study of the main technical and economical issues considered for the artificial lift system shift from RP into PCP in Teca and Nare fields located at the Middle Valley of Magdalena river Basin in Colombia.
Considerations for the shift system
Since its initial exploitaion (early 80`s) in Teca and Nare fields, Rod Pump (RD) was implemented together with cyclic steam injection as EOR to produce an oil of 12 °API and 12000 cp viscosity within heavy oil pattern.
On 1st of April of 2004 in Omimex Colombia (operator of the fields) a project of well description started and were identified a set of wells no suitables for steam injection due to conditions as high water cut, completion problems like collapsed casings, liner ruptures and high sand content at wellbore as well as low injectability factor.
A trial of PCP system on well Teca 326 started on 10th of January of 2005 with promising results on operational consitions and steady production of 50 BPD compared with the former RD.
Based on these results arose the idea to install 75 PCP systems on the set of wells with non injectable factibility.
According to the production rate 20 to 60 BPD (32 wells), 60 to 100 BPD (25 wells) and 100 to 150 BPD (18 wells) of the set of wells, three differents systems of PCP were designed with power of 10, 20 and 30 HP to cover respectively.
While installation of the new systems and period after an evaluation process and comparison, of performance and economics was done between the two systems. The results gives the following conclusions.
Technical issues evaluated were flow and viscosous fluid handling and specially energy consumption.
In this paper, we will describe techniques used to overcome the problems faced on the hydraulic fracturing jobs during the oil-to-gas conversion campaign in the Pilar field, Brazil.
The increasing demand for clean-burning natural gas in the Northeast of Brazil is fueled by the region`s industrial growth over the recent years, and represents the main drive for the oil-to-gas conversion campaign witnessed in the wells previously producing at marginal oil rates from the Coqueiro Seco formation in the Pilar field. These old wells are now producing gas from the 3000 meters deep Penedo sandstone formations. One of the main steps to meet the goals for natural gas output in the Pilar field was the hydraulic fracturing campaign in the deep Penedo formation. The treatment design and execution process to create these fractures was quite distinct from the normal jobs aiming at increasing the oil productivity in wells producing from the shallow Coqueiro Seco formation.
The Barra de Itiúba gas-bearing formation in the Furado, São Miguel dos Campos and Cidade de São Miguel dos Campos fields was also included, to a lesser extent, in the stimulation campaign aiming at the increase in natural gas production.
This paper describes the completion strategy for the old wells converted from oil to gas producers, highlighting the problems faced and overcome during the hydraulic fracturing campaign. In deviated wells crossing the deep Penedo reservoir, the risk of multiple fractures and influence of tortuosity have been diminished through corrective techniques, unique for each one of the existing wells. In the early hydraulic fracture treatments performed in the Pilar field, premature screen-outs were commonplace, disencouraging the use of the technique. The need to produce gas brought new ideas to the battlefield, and their implementation led to results beyond expectations.
The intense investiment to increase production of natural gas in the Alagoas, and its export through an expansion of the domestic natural gas transport network in the Northeast of Brazil, has the objective to keep up with the rapid growth in the regional gas consumption, due to an increase in the natural gas fired electricity generating capacity. Natural gas demand in Brazil, 1600 million scf/day in 2006, is expected to reach 4300 million scf/day by 2010, the direct result of investments in the sector estimated in US$ 22 billion.
The natural gas processing plant in Pilar was built to develop the compressed natural gas market for automotive, residential and commercial use in Alagoas, and to export gas through the Pilar-Cabo pipeline. Before its construction, all natural gas produced in Pilar and its neighboring fields was exported for processing and pumped back in the form of liquified petroleum gas. Seventy million scf/day of natural gas are processed in the Pilar plant, producing liquified petroleum gas, industrial gas and gasoline.
Given the increased demand for natural gas in the region, the gas-bearing formations in the Pilar area became attractive targets. Hydraulic fracturing played a major role in converting old oil wells producing at marginal rates from the shallow Coqueiro Seco reservoirs into good gas producers.
The Pilar Field
The Pilar, São Miguel dos Campos, Cidade de São Miguel dos Campos and Furado onshore gas and oil fields are located in the state of Alagoas, in the Sergipe-Alagoas Basin, in the Northeast of Brazil (Figure 1).
The Pilar field, discovered in 1981, is located near the city of Pilar (Figure 2). The Pilar field is characterized by intense compartmentalization produced by deltaic sedimentation that resulted in a stacked package of more than one hundred pay intervals, and by the extensional tectonics that produced a large number of fault blocks (Figure 3). The deposition occurred during the rift phase of the geologic evolution of the Sergipe-Alagoas Basin, in the Lower Cretaceous.
Intercalations of deltaic sandstones and shales compose the Coqueiro Seco formation, found at depths ranging from 500 to 2500 meters. These oil-producing sandstones have porosity of 20% and permeability of 100 mD.
As the emphasis on safety and efficiency increases in the oil industry, there has been a corresponding improvement in the quality of packaging for chemical products. Examples of higher standards include plastic shrink-wrap covers, better-designed pallets, and fewer sacks loaded per pallet. These improvements in packaging, however, have led to an increase in the volume of waste generated and its disposal costs.
In order to contribute to the preservation of the environment, improve employee safety and at the same time reduce operational costs, the HES Colombian environmental team designed the Integral Waste Management System (IWMS). The IWMS provides methods for efficient waste collection, recycling and disposal. The primary focus of the system is the provision of comprehensive environmental training for field personnel to help ensure that each person is fully educated concerning the process and its objectives. Formal standards for field applications are in place. One example of rig site implementation of the IWMS includes using size and dimension appropriate color-coded containers of the appropriate sizes to transport waste materials to the stock point.
Simultaneously with the intensive training undertaken by the personnel, the IWMS incorporates an equally important process for selecting the best local companies for recycling and disposal. Those companies selected received the guidance and support necessary to obtain and maintain ISO 9000 certification, assuring the quality of the service and the internal standards expected by our customers. All the selected companies are also certified by the Environmental Minister of Colombia.
Since the beginning the IWMS implementation in Colombia, waste incineration has been reduced nearly 90%, which helps decrease environmental pollution that may contribute to global warming. The IWMS efforts have assisted with the promotion of cardboard recycling companies and similar waste management enterprises. Most importantly, the cost of drilling waste disposal in participating Colombian operations has been reduced almost 60%.
During the last years in Colombia and in the whole World the natural resources care initiative has been created in order to improve the mankind living conditions. For that reason, there are well known processes as recycling, by means of which waste can be used in new and different products without using of environmental resources as Raw material
This concern has been reflected on the new Colombian environmental regulation which emphasizes the environment importance in this way: "Taking into account the general worry - about maintaining and improving the environmental quality and protecting the human health- is increasing different kinds of organizations are focusing their attention on their activities, products and services potential impacts??.
Considering this World initiative, Baroid team found out that the whole waste was incinerated. This fact generated a lot of costs; Approximately USD 8.000 per month was paid.
This paper describes a new method to squeeze perforations using non toxic chemical products. The product, belonging to the cyanoacrilate family (Ref 1), reacts with connate water dramatically reducing rock permeability in the injected intervals. Some technological solutions using this methodology are discussed. A variation of this methodology can be used to reduce or control sand production in low consolidated sandstones.
At the present time, useless perforations are plugged by squeezing cement into the perforating tunnel. The proposed alternative method is to eliminate fluid conduction through the perforations blocking the permeability in the surrounding formation. To obtain this, non toxic, low cost chemical products are injected to react with connate water, generating a mechanical resistant structure with negligible permeability. A simplified alternative allows for sand control. In this case the proposed methodology the products react partially with connate water, bridging together the sand grains. The chemical product is similar to that used to squeeze the perforating, but at lower concentrations. Several lab tests have been run to check the principles and limits of the chemical products used.
Consequently, it is now available a new system to plug useless perforation by injection of a chemical product, at low cost and operationally safe.
Using this new methodology it is possible to minimize cost and time during workover operation.
At the present time, the most used method to communicate hydraulically the production casing with net pay is the perforating.
To do this, shaped charges with different sizes, configurations, phasing and orientations are used, according needs determinated by reservoir engineering. Also the proper methodology, like under balance or overbalance is selected according needs. The entrance hole is in the range of 0,3 to 0,5?? (0,762 cm to 1,27 cm ) diameter, and the average penetration is in the range of 12 to 20?? (30,48 to 50,8 cm).
Sometimes, due to different reasons, it becomes necessary the hydraulic isolation of some already perforated zones.
The system used, at the present time, for hydraulic isolation of the perforating is filling all and each one of these with cement slurry (Ref. 2). The cement should be low permeability, low filtrate and controlled set according temperature and well conditions.
The volume of the tunnel generated by the shaped charges is low (in the order of few cubic centimeters), and must be filled by these cement Fig.1. The cement volume to be use should be at least the perforating volume, plus de filtrate volume, plus an operating volume to fill the casing shooted interval plus an excess to carry all these volume and squeeze under a pressure below the formation fracture and high enough to displace the liquid and or debris in the tunnel.
The squeezed slurry penetrates the perforating, displacing gradually the liquid in there, as the hesitation pressure forces the slurry to fill up completely the tunnels. This leads to the typical variation of pressure versus time in a normal cement squeeze operation, where the operation is considered normal when the final hesitation pressure, below formation fracture pressure, remains constant.
Different methods are used to place the cement slurry in the perforating zone: Balanced plugs and squeeze the plug; cement retainers bridge plugs; packers alone or joined with recoverable bridge plugs and well known combinations.
All this operations require a minimum slurry cement volume of 30 sacks and the use of different kind of down hole tools. As surface equipment, a minimum of a bulk truck and a cementer to mix and pump the slurry and the displacements is needed.
After the squeeze operations the excess cement should be drilled to continue the operations.
The method proposed in this paper has the following objectives:
Castanhal is an onshore heavy oil field located in Sergipe-Alagoas basin northest of Brazil. It is a shallow unconsolidated sandstone reservoir. It has 75 wells where the average reservoir depth is 350m. The oil has high viscosity ranging from 1000 cp to 9000 cp and API gravity ranging from 10° to 16° API.
In early eighties, a small steam injection project was started in the field, but due to operational problems it was interrupted few years later. In that time the oil low prices make the field be practically abandoned: The production was carried on by very few wells without any fluid injection. In 2003, some successful experiences with frac pack and horizontal wells lead to a renewed interest in the field.
Geological and numerical studies have been accomplished and a permanent temperature monitoring technology was selected to improve the reservoir knowledge and validate the studies.
Among the monitoring technologies available in the market DTS (Distributed Temperature Sensing) was selected. It
allows a complete wellbore-temperature profile in few minutes if needed. In this case four observetion wells were equipped with an optical fiber placed along the entire length of the well.
This paper will present a steamflooding pilot in a nine spot, with four temperature observation wells completed with frac
pack to avoid sand production and DTS to improve the understanding of steam breakthrough in the producer wells
and the steam path in the injection well. The information support better decision making to increase steam injection
Castanhal field was discovered in June/1967 by 1-CL-1-SE well. It is located in Brazil and lies on north of the Sergipe-Alagoas basin and It is 50km far from Aracaju city (Figure 1). Its reservoirs are fine-grained sandstones and conglomerates from the Carmopolis Member of the Muribeca Formation, with high permo-porosity, saturated with biodegradeable oil, high viscosity ranging from 1,000 to 9,000 cp and API gravity ranging from 10o API to16oAPI. The oil in place (OIP) is about 178x106 bbl (december/2006).
Figure 1: Castanhal field map location.
The field produced oil by cyclic steam stimulation (CSS) and steamflooding during 1990 year and due to lower Brent prices and high operational costs related to sand face control the steam injection project was abandoned2. In 2001 this situation changed by a well succeded frac pack project with equipaments suited for steam injection. A new steamflooding project in a nine spot was started in june/2006 but now adding distributed temperature monitoring in four observation wells.
The geological interpretation of the well logs in about 73 wells drilled allowed, with confidence, to map four pay zones
MUR/CPS-1, 2, 3 e 4. The Figure 2 shows a well log from Castanhal field with its four zones.
Operationally Castanhal field is divided in four sandstone zones, CPS-1, CPS-2, CPS-3 and CPS-4 with different
geological characteristics. Zone CPS-2 has the best lateral continuity followed by zones CPS-3, CPS-1 and CPS-4, where this last one is more affected by the presence of shales.