a guid iding lig light to cost- effective reserv rvoir monitoring - PowerPoint PPT Presentation

4D Gravity and Subsidence: a guid iding lig light to cost- effective reserv rvoir monitoring Egypt-Norway Technology Days

Outline • About OCTIO Gravitude • The principles of the technology • 4D gravity • Whole-field subsidence monitoring • Case studies: the value of gravity and subsidence • Concluding remarks 2

About Octio Gravitude 2012 Late 90’s 2013 GRAVITUDE technology developed IP and competence created within Statoil transfer by Statoil • Value of the technology proven in many fields on the NCS • Gravitude performed 8 surveys in 6 fields since 2012 • Best results to date • Safe operations with no HSE incidents • Ormen Lange field (operated by Shell) surveyed since 2012 • Octio has ten years of operational experience • Highly skilled technical team with diverse background 3

The surveys in a nutshell Concrete platform Sensor frame with 3 gravimeters 20’ per measurement and 3 pressure sensors Repeated visits Primary measurements: gravity and pressure at the seafloor ROV 4

Gravity and subsidence monitoring on the NCS Seafloor 1st # of Reservoir Area N. depth Field Comment surveys (km 2 ) survey depth (m) stations (m) Troll 1998 320 1400 30 x 50 113 Norway’s largest gas field 6 Mikkel 2006 230 2500 3 x 12 21 Smaller, deeper reservoir 4 Sleipner 2002 4 80 800 / 2350 4 x 10 50 Gas production + CO 2 injection Ormen Second largest gas field in Norway 2007 295-1130 2000 15 x 50 120 5 Lange Challenging oceanography, Shell-operated Oil field, subsidence is the main Statfjord 2012 140-200 2750 5 x 25 53 2 motivation Midgard 2006 240-310 2500 10 x 20 60 Deep reservoir 4 Snøhvit / 2007 250-340 2500 20 x 20 86 Gas production + CO 2 injection 2 Albatross 5

Gravity for reservoir monitoring • Sensitive to changes of density in the subsurface • It allows monitoring movements of fluid interfaces. Example: sea sea gravimeter gravimeter gas gas water from aquifers Before production start After production start • 𝜍 𝑥𝑏𝑢𝑓𝑠 > 𝜍 𝑕𝑏𝑡 ⇒ Δ𝑕 > 0 observed • Magnitude proportional to the raise of the contact • Spatial distribution of Δ𝑕 tells about comparmentalization, permeability, aquifer strengths 6

Some applications of gravity Unproduced Volume of compartments? gas in place? Acquifer strengths? hydrocarbon water 7

Accuracy of gravity at the seafloor m Gal 10 000 Satellite altimeter Airborne 1000 Shipborne Seafloor 100 Seafloor 1998 2002/2005 Land 10 2006/2007 2009 Borehole 2015 2013 Stationary 1 Year 1 µGal means: • 10 -8 m/s 2 , or 10 -9 g, or 100 kg at 0.8 m • Sub-meter sensitivity in a gas-water contact 8

Value of the gravity data Well data Seismic Increased recovery Update actual Aquifer volume of Infill well strength hydrocarbon planning reserves Lateral Installation of Gravity compart- Prediction of water compression mentalization breakthrough facilities Reservoir Understanding Planning of parameters reservoir behavior production, away from wells (e. g. permeability) pipeline use • At a cost ~15% of that of time-lapse seismic 9

Subsidence Measured through time-lapse changes in water pressure (after applying tide and environmental corrections) Subsid idence 10

The value of subsidence data Compressibility, pressure depletion Reservoir Lateral compartmentalization Drilling window for infill wells Subsidence Aquifer Compressibility, permeability (or uplift) Geological model Overburden Caprock integrity Installation safety and integrity • Accuracy on subsidence (published values): • Mikkel 2006-2011 time-lapse: 2.5 - 3.7 mm • Sleipner 2002-2005 time-lapse: 2.3 mm 11

Example: Troll field Survey setup • Pressure and gravity measured at each of the concrete platforms (typically 20 minutes) • Tide gauges deployed during the whole survey at a subset of stations • Allow to refer all pressure measurements to normal sea and atmospheric conditions • Enables comparison of data from different vintages • In all: • No storage issues : < 2 Gb per survey • Fast turnaround: three months from survey to final report • Environmental friendly 12

Field cases 13

Troll 2002-2009: subsidence Zero-level stations Pore compressibility: • Troll West PDO (1991): ~ 80·10 -5 /bar • Revised core data (2000): ~ 9·10 -5 /bar Maximum ~1 cm / year • Subsidence history-matching: ~ 3·10 -5 /bar Alnes, H., Stenvold, T. and Eiken, O. [2010] Experiences on Seafloor Gravimetric and Subsidence Monitoring Above Producing Reservoirs, 72nd EAGE Conf. and Exhib. , L010. http://www.slideshare.net/Statoil/alnes-et-al-gravity-and-subsidence-monitoring 14

Troll 2002-2009: 4D gravity Corrected for gas takeout Oil production • Significant raise of ~2 m of the gas-water contact observed in some stations already in 2002-2005 • Time-lapse seismic lines shot in 2002 and 2006 showed no evidence of the raise yet • 5-10 m were required for it to be visible over effect of pressure drop • Updated aquifer strengths in the reservoir simulation model http://www.slideshare.net/Statoil/alnes-et-al-gravity-and-subsidence-monitoring Alnes, H., Stenvold, T. and Eiken, O. [2010] Experiences on Seafloor Gravimetric and Subsidence Monitoring Above Producing Reservoirs, 72nd EAGE Conf. Exhib , L010. Gas production, movement 15 of the water front

Mikkel 2006-2011: 4D gravity Forward-modelled gravity change (µGal) • Initial uncertainties on: • External aquifers Data – model discrepancy  Less water than expected • Total volume of gas  More water than expected • The system has low tolerance for water production due to risk of hydrate formation • Results show lower water influx than expected • Aquifer volume reduced by factor 4 • Mikkel contains more gas than expected • This input enabled: • Better prediction of water breakthrough • Better long term planning From Vevatne, J. N., et al., [2012] Use of Field-wide Seafloor Time-lapse Gravity in History Matching the Mikkel Gas Condensate Field, 74 th EAGE Conference & Exhibition , Extended Abstracts, F040 16

Midgard 2006-2012: 4D gravity and subsidence • Uncertainties in the model: • Aquifer support and drainage patterns • Fault distribution and compartmentalization • Learnings from 4D gravity and subsidence: one segment underproduced, indicating sealing faults • Reinterpretation of available seismic data • Updated reservoir model with the sealing fault as the most likely realization • A new well target was identified, which is now number one producing well in all the Åsgard complex Ueland, I., et al., [2015] Modnet fram brønn på utradisjonel vis, Origo Statoil ASA, pp. 40-41 September . 17

Forward-modelled gravity change (µGal), Ormen Lange 2 years time-lapse Measured 2012-2014 • Increased recovery involved decisions on: • Compression facilities Significantly less • Infill wells subsidence than • Water break-through and modelled in the compartmentalization identified as a south Outtake significant uncertainties • Feasibility studies for measuring water influx: • Nearly impossible with 4D seismics • Would take > 7 years with EM Water • Abstract submitted to EAGE 2017 with influx Gravitude and Shell authors Dunn et al., A long-term seafloor deformation monitoring campaign at Ormen Lange gas field , first break volume 34, October 2016 Van den Beukel, A. et al. [2014], Integrated Reservoir Monitoring of the Ormen Lange field: Time lapse seismic, Time lapse gravity and seafloor deformation monitoring, The Biennial Geophysical Seminar, NPF, Kristiansand 2014 18

Gravitude’s feasibility studies Define alternative scenarios reflecting: • Uncertainties on reservoir model • Value of the new information Optionally Model the strength of 4D gravity and Environment conditions Requirements from • Oceanography subsidence signals for each alternative installation safety on • Seafloor quality scenario subsidence precision Feasibility report • Ability to distinguish reservoir scenarios Value of the gravity and subsidence data • Increased safety from subsidence monitoring • Survey design and cost 19

Conclusions • 4D gravity - subsidence surveys provide: • Key information for reservoir management, e.g. • Movement of fluid contacts • Identification of non-producing compartments • Reservoir compaction • Information for whole-field – not only producing wells • Improved safety of the field and installations 20

Thank you hugo.ruiz@octio.com 21