Oliver J. Kingsbury
Consultant
Pembroke, Wales
There are regions of the world where the only viable means of conducting a seismic survey on land must involve helicopters as a primary means of transport.
The high operating cost of helicopters means that such heliportable work is expensive compared with the more common land and marine surveys.
This article is addressed to exploration companies con- templating heliportable seismic surveys. Its aim is to show how these operations work and to enable a dramatic reduction in the cost and timescale of future operations compared with numerous operations this writer has witnessed and been involved with in recent years.
The core of this article concerns distinct designs of drilling machinery used in these activities and the most efficient ways of configuring this machinery in the field.
HELIPORTABLE CREW
The mobile sections of a heliportable seismic crew - survey, drills, and recorder are moving from left to right along a seismic line (Fig. 1) (28333 bytes).
Assuming that at least one helicopter is being shared between these sections, the sections should be kept as close as possible to each other to minimize helicopter usage in flying between sections.
Base is sited at a fixed position, serves as a logistics and communications center, and acts as a point for helicopter fueling and servicing.
Survey consists of a team marking the ground with a precise outline of the prospects' seismic lines as determined by the client.
Drills consists of a series of drilling rigs drilling bore- holes in positions previously marked by survey. Explosives are loaded into these holes to form the seismic energy source used in these operations.
Recorder detonates the explosives placed by drills and records raw seismic data.
DRILLING OPERATION
The emphasis of this article is on drilling machinery and the way it is used (Fig. 2) (30335 bytes). The deeper a borehole is drilled, the more explosive it can usefully contain and the greater the data coverage it can deliver.
Rig type A drills one relatively deep hole at a single location and then moves to its new location. When loaded with explosive, the hole becomes a shot point.
Rig type B drills a series of relatively shallow holes either side of a single location and then moves to its new location. Holes drilled by rig type B become shot points by loading them with explosives and linking defined groups in series to form individual shot points.
With rig type A, the entire rig stays in a fixed position while drilling at the single location, whereas the crews of rig type B must carry the drilling unit across the ground to reach drilling sites either side of the engine/ compressor unit.
The important distinction between these designs of rig is that type A rigs direct their operating effort solely into the ground, whereas type B rigs direct their effort into the ground and across the ground. Type A rigs are also heavier, more expensive to manufacture, and require a more skilled crew than type B rigs.
The most efficient way of configuring type A rigs is to space a series of these rigs as far apart from each other as possible. The theory behind this is that in a given length of line the fewer drill movements that take place the less helicopter use is needed to support those moves.
Successful operations using type A rigs have been carried out in this way for decades, a situation that appears to have led to the pervasive belief among many in the exploration industry that this method represents the most efficient way to conduct heliportable operations using any design of drill rig in any terrain.
In many cases, the efficient method of configuring type A rigs has been applied to operations using type B rigs, directly resulting in production delays and needless expense.
Unlike type A rigs, the efficient use and rate of production of type B rigs can be greatly influenced by the nature of the terrain on which they operate. Terrain discontinuities (Fig. 3) (25539 bytes) represent any feature that presents difficulty or impossibility for the drill crews to cross on foot. The convergencies represent the positions where the horizontal coverage of adjacent rigs meets up.
Fig. 3 (25539 bytes) shows the rigs in an ideal situation, with each dis- continuity coinciding with a convergency. If the rigs in Fig. 3 (25539 bytes) were spaced farther apart, each rig would have the problem of the drilling unit having to be taken across a discontinuity. Crossing of discontinuities, whether on foot or by helicopter, leads to delay and expense.
Thus it can be seen that type B rig production rates are inversely proportional to the number of discontinuities within a rig's horizontal coverage. Put more simply, in the terrain in Fig. 3 (25539 bytes), the farther apart type B rigs are spaced, the slower they can drill a set number of holes.
As shown in Fig. 1(28333 bytes), the heliportable seismic crew is made up of separate but interlinked components, the performance of any one of which will have direct and/or indirect effects on the others. A heliportable crew operating without its drill section running at optimum efficiency will be compromised as a whole.
A further point of relevance to these drilling operations is the number of drills in line. Analysis of helicopter movements in moving a series of equally spaced rigs to a new, similarly spaced location (Fig. 4) (37491 bytes) shows that helicopter use in this instance is a function of the square of the number of rigs in line.
If adjusting the number of rigs in line, the implications for helicopter use should be fully appreciated and planned for beforehand.
Splitting the drill line into sections at line intersections can have merit in that two or three small sections will use less helicopter time than a single large section, though this is dependent on the distances between sections. Proper modeling of these instances can be a useful tool.
CASE HISTORY
A heliportable seismic operation took place on predominantly fiat-surfaced yet heavily faulted and eroded terrain ideal for the correct use of the lightweight single-drill-unit type B rigs in service.
The power rating of this particular design of rig meant that the maximum extension of air hose that could be achieved before internal frictional losses made drilling unworkable was around 500 m. So, according to the dictum that the best efficiency is obtained by spacing the drill rigs as far apart from each other as possible, the rigs were spaced 900 m apart, each rig covering 450 m on either side of the engine/compressor unit. The drill production target equated to a horizontal coverage of 600 m/day/rig.
At a rig spacing of 900 m the crews' production targets were not being achieved and helicopter use was excessive. Prototypes of the analysis outlined in this article were drawn up and a reduction in drill spacing was initiated. To coincide with the daily production target, the new drill spacing was set at 600 m. Immediately on implementation of the new drill spacing, drill production rates rose to 50%, accounted for by the fact that there were now 50% more convergencies in a given length of line than previously and therefore 50% more opportunities to deal painlessly with discontinuities. Daily drill helicopter use declined from previous levels, less helicopter time being needed to cross discontinuities, and the now-being-achieved individual drill production of 600 m/day meant that the rigs would be moved at the same time each morning, rather than at different times through the day before, which drew helicopters from other tasks and increased transit times.
Since the recording production rate had previously been limited by the drill production rate, the great improvement in the drilling production rate now permitted a similar improvement in the recording production rate.
A simple modification to drill spacing had made a sig- nificant difference to the crew's overall efficiency and had dramatically reduced its costs. The reduced drill spacing remained in place for the remainder of the operation.
FUTURE OPERATIONS
The financial consequences of running heliportable operations in an inefficient manner can be very large indeed.
Computer simulations can play their part in the planning of these operations but are of little value if essential factors have been omitted.
Heliportable seismic operations that use properly designed type B drill rigs, used in the correct manner in the right conditions, can offer distinct cost advantages over operations that use type A rigs. Operations designed, modeled, and implemented from first principles prove to be the most efficient and least complicated.