In the past, wildcat (exploratory) wells used to be largely unsuccessful with so many dry holes drilled. (Learn more about dry holes in Dry Hole Explained in 6 Common Questions and Answers.) 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; we still need to know how big the reservoir is. Knowing how big the reservoir is opens up further questions, such as:

• Can the hydrocarbons in this field justify the cost of developing the field?
• What are the physical properties of the reservoirs (e.g., depth and thickness)?
• Where is the oil-water contact?

## The Steps to a Field Appraisal

This is where the field appraisal comes in, which aims at proffering solutions to our burning questions about how commercial the hydrocarbon in the field is. The 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 factor in economic terms to answer the question: "Are the hydrocarbons in this field enough to pay the bills to recover the money spent and still make us some profit?"

## Step-out Drilling

Remember 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. 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 pencil placed in one corner of the table. To do this you begin running your hands across the table until you find the pencil. Yippee. With step-out drilling, we are not trying to find a pencil but we are trying to find the oil-water-contact. This oil-water-contact gives us an indication of where the reservoir ends, i.e., the reservoir size and shape.

There can be 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 well is drilled further away from the other with the aim of knowing just how big the reservoir is.

We can also obtain information such as the reservoir thickness, shape and contours. Unlike the blindfold example, we are not entirely blind when doing this; we work with data from seismic surveys to give us some direction. (Discover some sources of information in the article Is Big Data Transforming the Oil & Gas Industry?) All these wells in step-out drilling are called delineation wells.

## Well Spacing

Figure 1. Diagram illustrating the concept of well spacing.

Even step-out drilling has to be planned. Planning the distance between well A and well B is known as well spacing. The well spacing for a gas reservoir is different from the well spacing for an oil reservoir. We plan our well spacing differently based on the kind of oil in an oil reservoir.

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 pressureand make it difficult to optimize recovery. Placing wells too far apart will cause us to poorly drain some areas.

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 a 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 more wells are required 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

Infill drilling is also used in the 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 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 the appraisal phase and the development phase may not be clearly defined. Especially for onshore fields, appraisal and development can go on almost at the same time; meaning that we can be appraising while concurrently producing from the wells drilled during a prior appraisal. 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 if the new well cannot drain enough hydrocarbons to justify the cost of drilling the well in the first place.

Conclusion

After striking oil and/or gas in an exploratory drilling, the field appraisal answers questions including 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, appraisals can almost go on with the field development phase. This may not be so with deep offshore fields because of their higher 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.