Maximising your gas potential

Dr Elmer Dougherty gives O&G the lowdown on planning and managing natural gas operations.

Presently economic luster of natural gas (NG) is dimmed by slackened demand overshadowed by excess capacity. In Canada and the US NG’s Btu-equivalent price has slumped to less than US$25/bbl. Thus, soundly financed companies have squeezed NG investment while leveraged explorationists are riding out a life-or-death struggle. Elsewhere, NG’s price-tie to oil supports a more limited restraint.

Nevertheless, certainty calls from the gloom, “This too shall pass”. The greenness halo over ‘clean’ NG with its maximal H/C ratio propels its use. Oil exporting countries can preserve the ‘golden-egg, laying goose’ by using NG for their prodigious power needs. NGL and gas-to-liquid plants provide growing consumer access. BP World Energy 2009 shows NG reserves (6534 TCF) equivalent to oil (1258 BBbl). With exploration’s focus – plus new tech a la shale gas – NG will expand.

Since its incorporation 30 years ago, Maraco has evolved a suite of software spanning NG exploration, development, and production. Our GORE (GasOilReserveEstimate) is tailored to address the explorationists key question, “How big is my find?” using Q&P from a production test GORE estimates GIP & decline fraction – and, given price and well cost, payout time for the next well. Updates with the latest observations provide the earliest guide to ‘what next’?

Simulation
GMAN.OPT simulates NG reservoir/surface flow while determining an optimal schedule of wells & compressors that stretches a plateau rate through time. Each addition to the schedule is the well/compressor that now has the highest return (PVR). Development stops – and decline begins – when PVR of all additional candidates is below cutoff. Experience has shown us that case-study schedules contain sub-standard investments. Years ago, a development plan prepared by a competent consultant – commissioned by client management as a check on OPT – carried 25 percent higher cost for the same rate profile. When price of liquids collapsed in 1986, this client eliminated a third of budgeted wells in a two-week evaluation.

Maraco’s GOMAN replaces OPT when:

• Detailed economic tables – revenue, cost and return – are needed
• Surface processing unit capacities – separators, sales gas, sulfur, NGL plants – and product compositions need detailing
• Number of reservoirs becomes large. The largest current application has over 70 reservoirs.

GOMAN also can integrate combined crude oil and associated and non-associated gas complexes, which is now being done for Saudi Aramco and Kuwait Oil Co.

Analysis
Finally, our GasPal system provides the most advanced nodal analysis NG planning tool available today. GasPal’s core program calculates capacity of one or many reservoirs flowing to one or several offtake points. The underlying framework for the model’s reservoirs is wells in place or scheduled versus time (and for layered formations perforation and plug dates or flow condition) and compressors dated-in at specified surface-network nodes. From capacity, GasPal determines production schedule with specified spare capacity pursuant to a requested rate profile. Aquifer influx, relative permeability, and gas trapped are accounted for. Elaborate graphics support history matching and performance interpretation.

Auxiliary programs are:

• UPSCLR – upscales a petrel geologic model into a GasPal computing grid via an onscreen interface
• GRIDDER – generates a computing grid onscreen overlaying a reservoir’s contour map
• TFR (TubingFlowlineRiser) history matches well/flowline performance using a collection of correlations.

A single reservoir model can be built and onstream in one hour (with run time for 360 months of 1-2 seconds). Large models requiring extensive history matching take longer. The development and validation of TOTAL’s Tunu Field model – 23 platforms, over 700 wells – stretched over several months. Similarly, for EBN’s (Dutch government) model of the Groningen Field. Here, wells produce into a ring of pipe. As regional demand shifts, gas-flow direction in the ring changes. GasPal is the only commercially available software that dynamically calculates such flow reversals in a pipe loop.

Dr Elmer Dougherty, founder of Maraco, develops and applies software to optimise oil and gas development and analyse production operations. Middle East consulting includes Saudi Arabia (Aramco), Kuwait (Kuwait Oil), Libya (Oasis) and Iran (Consortium). He is Emeritus Professor at the University Southern California, where he has 24 years petroleum and chemical engineering experience.