K.V.Kudashov, R.A. Malakhov, Rosneft Oil Company
In 2010, Rosneft was the fourth largest oil producer in the world, and the largest producer in Russia (Fig. 1). It is impossible to increase and maintain this position and continue to produce high production levels without increasing the volume of wells drilled and sidetracked. Production from these activities makes up over 10% of Rosneft’s annual oil production.
Rosneft is also the leading Russian company for drilling efficiency in new wells (Fig. 2). In 2010, production from Rosneft’s new wells was double the total national average. This is supported by:
» Reserves quality
» Continuous improvements in well completion technologies
» Broad applications of geological and technological model planning
» Proprietary methodological and program developments.
Any oil and gas company’s drilling program is based on the approved design solutions which are updated as each new well is drilled in every field. When preparing the well design documents, Rosneft uses a broad range of specialized software, both commercial and proprietary. In particular, software packages of geological (Schlumberger Petrel, IRAP RMS) and hydrodynamic (Schlumberger Eclipse, CMG STARS) modeling.
After analyzing the field drilling results for the previous year, we have prepared a 5 year drilling program which is part of a key business plan for Rosneft and its subsidiaries. During such an update, it is important to include all geological and production data and their latest modifications. In order to resolve this challenge, when preparing a drilling and side-tracking program, our geological departments use “Geology & Production software” (G&P) solutions developed by the RN-UfaNIPIneft R&D institute with diverse functionalities in data storage, processing and visualization, and modules for analytical calculations.
When further drilling and sidetracking programs are implemented, continuous monitoring and adjustments are required. At the implementation stage, the departments responsible for drilling support use, in addition to our “G&P” software, the “Production Technological Information System “ (TIS) developed by the company, which uses the most up-to-date data sources available.
As a result, the entire array of geological and geophysical data accumulated during the previous drilling and side-tracking programs is used within the geological and hydrodynamic simulations used to design the future drilling programs. Figure 3 shows how the concept was implemented at Rosneft.
Rosneft has also developed the “Well Construction Control and Management” system (WCCM) with the purpose of reducing capital expenditure on high quality wells, and to provide a unified system for well construction, control and management. One of the main elements of this package is the Drilling Remote Monitoring (DRM) system, a set of data collection software that ensures the correct data processing and well construction database, which integrates all the system elements (modules). At the same time DRM ensures the real-time transfer of geological and technical parameters, collected in the wells’ construction process.
It should be noted that this was the first instance that such a solution was used in Russia, when Rosneft developed the Horizon software package for geosteering (geological support) of horizontal wells (HW) and sidetracks (HS) drilling in 2007. This software was unique because no commercial software in the market, at that time, solved the problem of targeting and drilling into the pay zone. Until recently this task had only been solved with help provided by the major service companies.
Geosteering of horizontal wells and side-tracking
The primary objective of geosteering and side tracking is to achieve the maximum well bore profile targeting the most productive zone of the formation, accounting for geological peculiarities and technical limitations. As a result the maximum possible production of recoverable oil reserves, at a maximum production rate, will be ensured.
A horizontal section of well bore’s effective length is the total length of the well bore that has contact with the reservoir based on the logging results. In this case the efficiency of well (sidetrack) geosteering is estimated as a ratio – Lef/Ltot – is the total length of a horizontal leg over the length from production casing seat (or liner cement collar) to the bottom hole.
In order to achieve maximum efficiency one should account for the following factors, which may be divided into two groups.
1. Geological features of formations in the drilling area:
» formation discontinuity and non-uniformity of its properties related to area and cross-section
» uncertainty of gas/water and oil/water contact positions
» recovery of reserves and phase displacement
» initial and current formation pressure.
2. Technical limitations applied to a specific well:
» maximum wellbore inclination
» maximum possible total depth of a well
» the effects of drilling fluids on the formation
» possible failures, breaks and wearing of the drilling equipment, tools and instruments affecting directional accuracy and control.
Successful (from geological and technical viewpoints) construction of a horizontal wellbore is only possible when the following conditions are met:
» availability of a strictly developed interaction system for the departments involved, and a clear decision making plan
» bedding prediction using the uncertainty analysis prior to and in the process of HW drilling
» consideration of geological irregularities and technical limitations for each field.
Horizon – the primary geosteering tool
The efficient horizontal wellbore geosteering is primarily influenced by two geometric factors:
» uncertainty of the reservoir’s position between the wells
» uncertainty of the wellbore’s measurements during drilling
The actual configuration of the formation may differ significantly from the existing idea of the formations structure even when detailed 3D geological models are available. There may be several reasons for this: deviations in the wellbore inclination measurements, extension or compression of the logging curves resulting from the configuration of intersecting formations or inaccuracies in the correlations of the borehole profile.
For low-angle wells, the inclination angle, if throughout the entire length of the well it does not exceed 90°, a regular correlation method may be used to identify the current position of the bottom hole in the profile. In the case of geosteering of HW and HS drilling, when the inclination angle exceeds 90°, it is not sufficient to apply a regular vertical layer-by-layer correlation.
When drilling directional wells, vertical incline deviations of 3-5m do not really influence the decision on whether to open up certain formation intervals and consequently, on reserve recovery and management. During HW drilling, especially in the areas with small net pay and in the zones of increased non-uniformity, a 1-2m vertical mistake in inclination measurements may adversely affect the way the target section is opened up with the horizontal leg and, consequently, will affect ultimate reserve recovery.
The uncertainty in the inclination measurements are related to:
» deviations of inclination and horizontal angles measurements (instrument accuracy)
» deviations of depth measurements (pipes tally)
» inaccurate north reference.
These deviations arise as a result of magnetized pipes and an overall magnetic environment, gyroscope drifting which depends on the Earth rotation and latitude, and the influence of the tool position in the wellbore.
In order to eliminate uncertainties related to the geometry of the formation and inclination measurements, one has to use methods that allow you to identify the current bottom-hole location related to the target formation, i.e. the intrawell correlation requires taking into account the formation dip and wellbore inclination.
Currently, during real-time geosteering, the most efficient method is a 2D synthetic seismic model (2D SSM) implemented by the “Horizon” software. This method is based on the creation of synthetic geophysical log along a side track, and its set up for actual logging recorded during drilling by matching the target formation dip and top relative to the horizontal wellbore. Therefore, the problem of identifying the sidetrack position in the profile, based on actual logging and inclination measurement is resolved.
Main assumptions of 2D SSM method:
» 2D task is solved, i.e. it is assumed that a target formation is laterally continuous and homogenous
» The logging data of the pilot or neighboring wells is used while it was recorded, without adjustment to the geometry of the formation intersection with the target interval
» The measurements of the horizontal leg inclination are assumed to be correct and all calculations are made based on the interpretation of inclination measuring as presented by contractor
» The apparent dip of the formation position in terms of the direction of drilling is to be identified.
The task of geosteering, and the 2D SSM method in particular, is not an accurate determination of the target formation geometry. The main objective is to identify the relative position of the formation in relation to the HW (HS) based on the inclination data, the logging results and the data produced from the pilot or surrounding wells, all accounting for the above assumptions.
The primary advantage of implementing the “Horizon” software in this approach is the ability to make real time decisions on trajectory adjustment during drilling, with minimum source data being available.
The number of new wells and sidetracks, drilled at Rosneft’s fields in 2006-2010, had increased by more than a factor of 2 and 7 respectively. The number of horizontal wells and side tracked wells increased significantly (in 2005 one horizontal well was drilled, in 2010 – 176 wells were drilled).
Considering the constantly increasing and changing scope of drilling, the efficient implementation of new technologies, while ensuring the high reliability of the drilling program is impossible without the latest IT technologies. In order to solve this, and other operational tasks, the company pays a lot of attention to the research and development of specialized IT systems. Currently, Rosneft uses modern software and information systems, including those of its own development, in the process of drilling program planning, implementation and monitoring.
The article was published in the Rosneft Scientific and Technical Newsletter (nauchno-technicheskiy Vestnik OAO “NK “Rosneft”) No.2, 2011, pp.16-19; ISSN 2074-2339. Printed with permission from the Editorial Board.