As a basis for energy management in apartment blocks, this paper characterises heat use in a large housing cooperative in Norway, with in total 1,058 apartments. Heat measurements with hourly resolution are available from 20 heating substations. Average specific heat delivery is 139 kWh per heated floor area. A linear regression model is described, modelling specific heat delivered to the apartments. Models are also used for separating heat to domestic hot water (DHW) from the total heat delivery, with a modelled DHW heat delivery of 34 kWh/m2. Daily heat load profiles are presented, for delivered heat during weekdays and holidays in the annual seasons. The study shows a potential for shifting heat loads in time on a neighbourhood level.

Summary Rock-compaction drive under waterflood re-pressurization has not been accounted for previously in our flow-model studies for a Valhall waterflood. However, field observations from pilot water-floods indicate an increase in permeability with the injection of cool seawater into the chalk formation. Platform subsidence measurements taken during the pilot waterflood also provide evidence of a chalk/water interaction. Laboratory experiments on reservoir core samples indicate an accelerated compaction effect as the flood front passes through the sample. To assess the value of a large-scale waterflood at Valhall, we have developed a new approach to simulate the possible effects of water-induced rock compaction in our black-oil flow models.

Summary Past years' forecasting performance has demonstrated that single "most likely" (deterministic) prediction of future production, without quantifying the associated uncertainties, is inadequate for management and planning purposes. Based on this an effort was initiated to improve the forecasting methodology and procedures with special emphasis on quantifying the uncertainty in short-term production forecasting (STPF). The Valhall field, offshore Norway, presented special problems for production forecasting because many unusual events affect production, even for STPF. The soft compacting reservoir chalk causes well collapses and chalk influxes and makes drilling difficult. Modeling of reservoir fracturing and therefore well performance is also uncertain. A probabilistic forecasting approach was adopted using a customized spreadsheet and commercially available statistical analysis add ins which allowed deterministic forecasting, partially probabilistic analysis, or fully integrated uncertainty analysis. Uncertainties were characterized by distributions based on historical data where possible. Communication and integration of knowledge were also key success factors since the process required input from several departments and many individuals to ensure that the "company knowledge" was fully reflected. A "common language" to communicate uncertainty and an auditable process to ensure buy in and consistency were also critical.

Abstract In the Valhall field in the North Sea, significant resources have been identified in the flanks of the field. In this situation the reservoir targets are the main driver for the wellbore trajectory planning. Even if all of the traditional drilling parameters have been addressed and the well can be drilled on paper, the wellbore can experience wellbore stability problems (lost circulation, stuck pipe, tight hole). These wellbore problems can gradually progress or suddenly occur. The problems may be aggravated by mud properties or mechanical interaction between the rock and the drilling assembly. Wellbore stability models, using offset drilling information, indicated early in the planning process at Valhall that the drilling of extended reach wells in the field was going to be a challenge. This challenge is the result of a narrow safe operational mud pressure window that narrows progressively with increasing wellbore sail angle. This theoretical window upper bound is given by lost circulation. The lower bound is given by the onset of wellbore collapse. This narrow window leaves hardly any room for the surge and swab pressures associated with pipe movement and pump changes. The flank locations require sail angles close to 75 degrees in the overpressured unconsolidated claystone of Tertiary age. The record well to date had a build and hold trajectory at 74 degrees. On this record well around 3 MM USD was spent on fighting wellbore stability problems (around 750 hours). Since traditional wellbore stability analyses have already indicated a high risk of instability, and some wells have been successful, while others not, consideration was given to developing new tools to assist in the well planning in the overburden. We wanted to look at potential heterogeneous rock strength changes that could not be picked up using offset well information in the planning. The approach was to develop a more detailed geological model for the overburden with geologic formation top surfaces and 3D seismic coherency data. In this 3D data cube the drilling problems encountered on the offset wells were correlated along the wellbores. The 3D data presentation was done in a standard visualization software (GEOVIZ). The paper will present the geomechanical theory behind wellbore stability problems in faultzones and fractured rock mass, how the geologic model has been developed and how the visualization software is used to select a wellbore trajectory with the lowest risk of wellbore instability problems. Several case histories are included to illustrates the potential this methodology has for reducing unscheduled events due to wellbore instability problems while drilling the wellbore from the seafloor to the reservoir. Potential for further improvement is highlighted

North Sea Central Graben chalk reservoirs are unique in that they have exceptionally high porosities at several thousand metres depth. This is partly an effect of overpressure, but, equally important are the combination of diagenesis, fracturing, and the timing of hydrocarbon generation, migration and entrapment. For chalk to serve as a reservoir, available porosity and permeability at the time of hydrocarbon migration are critical. Porosity in a normal chalk is reduced by early marine cementation, mechanical compaction and chemical compaction (i.e. dissolution and precipitation of calcite cement). To what extent these processes influence porosity evolution through time is related to the chalk’s burial history, specifically the pressure history, thermal history and the partial pressure of CO 2 (P CO 2 ). This was demonstrated by modelling porosity evolution through time in a cross-section extending from crestal Valhall Field to basin areas. The modelling results show that at crestal Valhall the chalk is highly porous and only mechanically compacted. Thermal and pressure (P CO 2 ) conditions were unfavourable for carbonate burial diagenetic cement to form prior to ≈20 Ma. At this point, oil emplacement prevented further porosity reduction, despite increasing temperatures and favourable P CO 2 conditions during continued burial. In basin areas, rapid burial and exposure to high temperatures combined with favourable P CO 2 conditions, from ≈40 Ma until present, resulted in more extensive porosity reduction by mechanical and chemical compaction. Permeability in chalk is significantly enhanced by fracturing. Chalk fractures if total stress (pore pressure + bending moment) is equal to or exceeds fracture pressure. Providing that fractures are open while hydrocarbons are migrating, these will serve as migration pathways. As for the modelled basin areas, absence of tensional fractures at the time of hydrocarbon migration prevented filling of potential reservoirs. Instead, hydrocarbons were directed laterally towards crestal Valhall where pore space and permeability enhancing fractures were available. By combining the results from the chalk diagenesis and fracture studies with hydrocarbon generation, migration and entrapment modelling, the distribution of known chalk fields and dry holes in the area was reproduced. The approach taken in the study emphasizes the necessity of a detailed understanding of the processes behind empirical observations such as porosity values in North Sea chalk reservoirs, a full evaluation of all factors having an impact on fracturing, and the integration of this understanding with basin-scale hydrocarbon migration models. Despite still unresolved questions concerning the chalk hydrocarbon system, the achievements so far are promising and encouraging for future prediction of hydrocarbon accumulations.

The depositional model for the Tor Formation at the Valhall and Hod fields has been revised to reflect the influence of syn-depositional faulting and reworking on reservoir thickness variations. New ideas for the structural and depositional model were based on detailed reservoir interval correlations in the Valhall–Hod area using biostratigraphy and graphic correlation techniques. Three-dimensional seismic data was also integrated and applied to develop the model. The Tor Formation was deposited during a tectonically active time in the Late Cretaceous and the Early Tertiary. During this time Valhall and Hod anticlines were forming as a result of inversion along the Lindesnes Ridge. Regionally the Tor and Hod formations thicken away from the Ridge. This indicates that the area was a structural high which is interpreted to be a large shoal. A major unconformity is present at the Hod/Tor Formation boundary. During continued uplift, crestal areas collapsed forming a series of horsts and grabens. The Tor Formation (Campanian and Maastrichtian, with documented Danian reworking) filled in the lows formed by these graben. Seismic data and well control demonstrate that the Tor Formation is thicker in graben and thinner over horsts. Removal of the Hod and Tor formations from local structural highs can be best explained by wave action and winnowing across a large shoal. The graben areas, sheltered from waves and protected from scouring and erosion, became local depositional centres for thicker slumped and reworked Tor Formation chalk. In the Tor Formation there is strong evidence for local erosion, formation of hard-grounds, grain size sorting, and down-slope movement through slumping and debris flows. Thickness and facies variations in the Tor Formation of Valhall and Hod fields can be linked to syn-depositional faults. Recognition of a fault controlled depositional model along with detailed mapping of thick reworked chalk in local graben led to the definition of additional reserves in the Valhall–Hod area.

Abstract Oil production from this chalk field has been improved significantly over the last three years through the use of multiple proppant fractures placed from horizontal completions. Following early successes the engineering effort now focuses on increasing value through engineering innovation and increased productivity. This paper describes, through case histories and field data, the advances made in the design and execution of these completions over the first 13 such wells in the field. Wellbore completion activities and stimulation are now performed as a standalone process through the use of specialized large diameter coil tubing equipment so allowing on-going drilling operations to be performed concurrently. This has significantly reduced daily spread costs while bringing wells on production within a much reduced time frame. This lower cost environment has also allowed innovative field procedures to be developed resulting in further improvements such as proppant plugs for isolation between stimulation zones and treatments using recycled proppant to minimize waste. Treatment design has been refined through downhole pressure monitoring which has allowed empirical frictional relationships to be developed for proppant slurry and coil tubing circulating fluids. This data validates the wellbore tubulars design and allows any weak points in the arrangement to be highlighted for change. An extensive fracturing database is maintained allowing each treatment design to be compared with previous experience in the field. Productivity is the driver for economic success and treatment design requirements have evolved through laboratory testing to account for longer term downhole operating conditions below the bubble point while also improving initial conductivity. Field data to date through PLT logging has confirmed the hydrocarbon contribution from each fracture and this is used to validate, or otherwise, the effectiveness of each treatment. Normalized well productivities continue to improve and the challenge is to maintain the rate of evolution through the improved application of both new and existing technologies. P. 335

Abstract Accurate production forecasting is a critical factor in an oil company's management and decision making. The production forecast forms the basis for long term planning and decision making, investment decisions and to set performance targets for the organization. For the Valhall field, offshore Norway, forecasting has proved to be difficult as a single discrete event, like the failure of a key producer, can reduce annual average production by up to 10%. The current activity level at Valhall is at an all time high, with two drilling rigs occupied with drilling of new production wells, adding up to 12 new, long reach horizontal wells to the production potential yearly. Accurate prediction of drilling time and initial well rates has proved challenging, adding significant uncertainty to the production forecast. Past years forecasting performance has demonstrated that single most likely' prediction of future production is inadequate for management and planning purposes. Based on this an effort was initiated to improve the forecasting methodology and procedures with special emphasize on handling of uncertainty in Short Term Production Forecasting (STPF). With the STPF being an ongoing effort throughout the year, the preferred methodology should balance the need for using techniques that adequately handle complex relationships while still being efficient and relatively easy to apply in the daily work situation. A spreadsheet based approach to the problem is described, where the standard parameters that impact the future production of most oil fields are handled, as well as the special challenges faced in predicting the performance of the Valhall field. The proposed methodology shows how a customized spreadsheet and use of commercially available statistical analysis ad-ins allows for traditional deterministic production forecasting, partially probabilistic analysis or fully integrated uncertainty analysis. The resulting forecasts assign probabilities to all outcomes as well as a ranking of the uncertainties in the form of a tornado chart. The approach is not unique, as some companies are using techniques based on similar concepts. The specific challenges at Valhall adds additional layers of complexity, and requires extensive integration of the various risk elements. In addition to having an effective forecasting tool, communication and integration of knowledge are seen as key success factors in order to improve the short term production forecasting. The production forecast requires input from several departments and many individuals to ensure that the "company knowledge" is fully reflected. A common language's to communicate uncertainty, and an auditable process to ensure buy-in and consistency is critical for a successful outcome, and is equally important when communicating results. This paper discusses both the development and use of the forecasting tool, and the forecasting process. P. 149

Abstract The combination of in situ effective stress and reservoir chalk mechanical weakness in the Valhall field has resulted in compaction and associated subsidence at the mudline. This paper summarizes recent finite element modeling of this phenomenon, emphasizing items of concern in a numerical simulation of this nature. The paper also presents new results from a simulation allowing inelastic overburden response. P. 377