In the past, wildcat wells used to be largely unsuccessful with so many dry holes drilled. Current technology has greatly reduced the uncertainty with exploratory drilling. Nonetheless, the only way to still confirm the presence of suspected hydrocarbon accumulation is by drilling a well. Furthermore, it is not enough to strike oil with an exploratory well (wildcat), we still need to know how big the reservoir is. Knowing how big the reservoir is opens up further questions like ‘Can the hydrocarbons in reservoirs in this field justify the cost of developing the field?’; ‘What are the physical properties of the reservoirs like depth and thickness?’; ‘Where is the oil-water contact?’
This is where field appraisal comes in. Field appraisal aims at proffering solutions to our burning questions about how commercial the hydrocarbon in the field is. Now, understand that field appraisal does not seek to confirm the presence of hydrocarbons; we have already gone through that in our exploratory wells. Our primary concern now is to know the size and shape of the individual reservoirs in the field, the quantity of hydrocarbons in the field and when we factor in economic terms, answer questions like ‘are the hydrocarbons in this field enough to pay the bills to recover dollar spent and still make us some profit?’
The different steps to field appraisal
Don’t forget that a reservoir is larger than a well and a field is larger than a reservoir. A successful wildcat well only confirms our suspicion of the presence of hydrocarbons in that area. Now, to answer the question of how big this particular reservoir is, we embark on something called step-out drilling. Consider this illustration; a friend blindfolds you and places you beside a table in a room, your task is to find a box of matches placed in one corner of the table. To do this you begin running your hands across the table until you find the matchbox. Yippee. Now with step-out drilling, we are not trying to find a matchbox but we are trying to find the oil-water-contact. This oil-water-contact gives us an indication of where the reservoir ends; reservoir size and shape.
Remember that we can have structural or stratigraphic traps. A trap is like a container and we want to know how big this container is, so we keep drilling one well after another. Well B is drilled some distance from well A, then well C is drilled closer to well B. We do this until we drill into water instead of oil and or gas. This is step-out drilling, more like drilling in steps; one further away from the other with the aim of knowing just how big the reservoir is. We can also get data like reservoir thickness, shape and contours from this. Unlike the blindfold example, we are not entirely blind when doing this; we work with data from seismic surveys to give us some direction. All these wells in step-out drilling are called delineation wells.
Even step-out drilling has to be planned. Planning the distance between well A and well B is well spacing. The well spacing for a gas reservoir is different from the well spacing for oil reservoirs. As a matter of fact, based on the kind of oil in an oil reservoir we can plan our well spacing differently. The purpose of well spacing is to position wells in points where they will efficiently drain the hydrocarbons in the reservoir. Placing wells too close to each other can quickly deplete the reservoir pressure and make it difficult to optimize recovery. Placing wells too far apart will cause us to poorly drain some areas. So well spacing is an important aspect of the appraisal phase and even runs into the development phase. Gas wells typically have 640-acre spacing between wells because natural gases are light and have high flow rate. Oil wells can have a smaller well spacing plan; it is common to find 160-acre, 80-acre and even 40-acre for oil wells. This is because you will require more wells to properly drain an oil reservoir than a gas reservoir with equal dimensions. Once again, it is not economical to just cramp wells too close to each other and at the same time we do not want to arbitrarily increase the distance between wells.
Infill drilling is also used in development phase. We may end up spacing wells too far apart and later find out that we are poorly draining the hydrocarbons in the reservoir. To fix this, we place another well in-between well A and well B. We can call this new well ‘AB’. Well AB is an infill well drilled into wells A and B. At this point it is important to note that sometimes, the divide between appraisal phase and development phase may not be defined. Especially on onshore fields, appraisal and development can go on almost at the same time; meaning that we can be appraising while producing from the wells drilled during appraisal concurrently. Infill wells can form part of the field appraisal procedure, however, the cost of an infill well has to be justified. There is really no need to drill an infill well when the new well drilled cannot drain enough hydrocarbons to justify the cost of drilling the well in the first place.
After striking oil and or gas in an exploratory drilling, appraisal answers questions like how big the reservoir is and if the hydrocarbons trapped in reservoirs in the field can pay the bills and still permit us to make some profit. In onshore fields, appraisal can almost go on with the field development phase. This may not be so with deep offshore fields because of cost and unique technical challenges. The development phase of a field can run into several decades, so it is important to take that time to appraise the field, gather more data and understand how best to recover trapped hydrocarbons. This article goes over the most common steps used in field appraisal to answer burning questions about the field.